Expand plugin host surface (#5205)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies
> - The plugin system is the extension boundary for optional product
capabilities
> - Rich plugins need more than a worker entrypoint: they need scoped
database storage, local project folders, managed agents/routines, host
navigation, and reusable UI components
> - The LLM Wiki work exposed those missing host surfaces while keeping
plugin code outside the core control plane
> - This pull request expands the core plugin host, SDK, server APIs,
and UI bridge so plugins can declare and use those surfaces
> - The benefit is that future plugins can integrate with Paperclip
through documented, validated contracts instead of bespoke server or UI
imports

## What Changed

- Added plugin-managed database namespaces and migration tracking,
including Drizzle schema/migration files and SQL validation for
namespace isolation.
- Added server support for plugin local folders, managed agents, managed
routines, scoped plugin APIs, and plugin operation visibility.
- Expanded shared plugin manifest/types/validators and SDK
host/testing/UI exports for richer plugin surfaces.
- Added reusable UI pieces for file trees, managed routines, resizable
sidebars, route sidebars, and plugin bridge initialization.
- Updated plugin docs and example plugins to use the expanded host and
SDK surface.

## Verification

- `pnpm install --frozen-lockfile`
- `pnpm run preflight:workspace-links && pnpm exec vitest run
packages/shared/src/validators/plugin.test.ts
server/src/__tests__/plugin-database.test.ts
server/src/__tests__/plugin-local-folders.test.ts
server/src/__tests__/plugin-managed-agents.test.ts
server/src/__tests__/plugin-managed-routines.test.ts
server/src/__tests__/plugin-orchestration-apis.test.ts
ui/src/api/plugins.test.ts ui/src/components/FileTree.test.tsx
ui/src/components/ResizableSidebarPane.test.tsx
ui/src/pages/PluginPage.test.tsx ui/src/plugins/bridge.test.ts` passed:
11 files, 67 tests.
- Confirmed this PR changes 89 files and does not include
`pnpm-lock.yaml` or `.github/workflows/*`.

## Risks

- Medium: this expands plugin host contracts across db/shared/server/ui
and includes a new core migration (`0076_useful_elektra.sql`).
- The plugin database namespace validator is intentionally restrictive;
plugin authors may need follow-up affordances for SQL patterns that
remain blocked.
- Merge this before the LLM Wiki plugin PR so the plugin can resolve the
new SDK and host APIs.

> For core feature work, check [`ROADMAP.md`](ROADMAP.md) first and
discuss it in `#dev` before opening the PR. Feature PRs that overlap
with planned core work may need to be redirected — check the roadmap
first. See `CONTRIBUTING.md`.

## Model Used

- OpenAI Codex, GPT-5 coding agent, tool-enabled shell/git/GitHub
workflow. Context window size was not exposed by the runtime.

## Checklist

- [x] I have included a thinking path that traces from project context
to this change
- [x] I have specified the model used (with version and capability
details)
- [x] I have checked ROADMAP.md and confirmed this PR does not duplicate
planned core work
- [x] I have run tests locally and they pass
- [x] I have added or updated tests where applicable
- [x] If this change affects the UI, I have included before/after
screenshots
- [x] I have updated relevant documentation to reflect my changes
- [x] I have considered and documented any risks above
- [x] I will address all Greptile and reviewer comments before
requesting merge

---------

Co-authored-by: Paperclip <noreply@paperclip.ing>

ui/src/components/Layout.tsx

@@ -1,6 +1,7 @@
 import { useCallback, useEffect, useMemo, useRef, useState } from "react";
 import { useQuery } from "@tanstack/react-query";
 import { Outlet, useLocation, useNavigate, useNavigationType, useParams } from "@/lib/router";
+import { CompanyRail } from "./CompanyRail";
 import { Sidebar } from "./Sidebar";
 import { InstanceSidebar } from "./InstanceSidebar";
 import { CompanySettingsSidebar } from "./CompanySettingsSidebar";
@@ -16,6 +17,7 @@ import { ToastViewport } from "./ToastViewport";
 import { MobileBottomNav } from "./MobileBottomNav";
 import { WorktreeBanner } from "./WorktreeBanner";
 import { DevRestartBanner } from "./DevRestartBanner";
+import { ResizableSidebarPane } from "./ResizableSidebarPane";
 import { SidebarAccountMenu } from "./SidebarAccountMenu";
 import { useDialogActions } from "../context/DialogContext";
 import { GeneralSettingsProvider } from "../context/GeneralSettingsContext";
@@ -39,9 +41,18 @@ import { queryKeys } from "../lib/queryKeys";
 import { scheduleMainContentFocus } from "../lib/main-content-focus";
 import { cn } from "../lib/utils";
 import { NotFoundPage } from "../pages/NotFound";
+import { PluginSlotMount, resolveRouteSidebarSlot, usePluginSlots } from "../plugins/slots";
 
 const INSTANCE_SETTINGS_MEMORY_KEY = "paperclip.lastInstanceSettingsPath";
 
+function getCompanyRouteSegment(pathname: string, companyPrefix: string | undefined): string | null {
+  if (!companyPrefix) return null;
+  const segments = pathname.split("/").filter(Boolean);
+  if (segments.length < 2) return null;
+  if (segments[0]?.toUpperCase() !== companyPrefix.toUpperCase()) return null;
+  return segments[1]?.toLowerCase() ?? null;
+}
+
 function readRememberedInstanceSettingsPath(): string {
   if (typeof window === "undefined") return DEFAULT_INSTANCE_SETTINGS_PATH;
   try {
@@ -83,6 +94,38 @@ export function Layout() {
   }, [companies, companyPrefix]);
   const hasUnknownCompanyPrefix =
     Boolean(companyPrefix) && !companiesLoading && companies.length > 0 && !matchedCompany;
+  const pluginRoutePath = useMemo(
+    () => getCompanyRouteSegment(location.pathname, companyPrefix),
+    [companyPrefix, location.pathname],
+  );
+  const routeSidebarCompanyId = matchedCompany?.id ?? null;
+  const routeSidebarCompanyPrefix = matchedCompany?.issuePrefix ?? null;
+  const { slots: routeSidebarSlots } = usePluginSlots({
+    slotTypes: ["page", "routeSidebar"],
+    companyId: routeSidebarCompanyId,
+    enabled: Boolean(routeSidebarCompanyId && pluginRoutePath),
+  });
+  const routeSidebarSlot = useMemo(
+    () => resolveRouteSidebarSlot(routeSidebarSlots, pluginRoutePath),
+    [pluginRoutePath, routeSidebarSlots],
+  );
+  const sidebarContext = useMemo(
+    () => ({
+      companyId: routeSidebarCompanyId,
+      companyPrefix: routeSidebarCompanyPrefix,
+    }),
+    [routeSidebarCompanyId, routeSidebarCompanyPrefix],
+  );
+  const companySidebar = routeSidebarSlot ? (
+    <PluginSlotMount
+      slot={routeSidebarSlot}
+      context={sidebarContext}
+      className="h-full w-full"
+      missingBehavior="placeholder"
+    />
+  ) : (
+    <Sidebar />
+  );
   const { data: health } = useQuery({
     queryKey: queryKeys.health,
     queryFn: () => healthApi.get(),
@@ -335,13 +378,16 @@ export function Layout() {
             )}
           >
             <div className="flex flex-1 min-h-0 overflow-hidden">
-              {isInstanceSettingsRoute ? (
-                <InstanceSidebar />
-              ) : isCompanySettingsRoute ? (
-                <CompanySettingsSidebar />
-              ) : (
-                <Sidebar />
-              )}
+              <CompanyRail />
+              <div className="w-60 shrink-0 overflow-hidden">
+                {isInstanceSettingsRoute ? (
+                  <InstanceSidebar />
+                ) : isCompanySettingsRoute ? (
+                  <CompanySettingsSidebar />
+                ) : (
+                  companySidebar
+                )}
+              </div>
             </div>
             <SidebarAccountMenu
               deploymentMode={health?.deploymentMode}
@@ -352,20 +398,16 @@ export function Layout() {
         ) : (
           <div className="flex h-full flex-col shrink-0">
             <div className="flex flex-1 min-h-0">
-              <div
-                className={cn(
-                  "overflow-hidden transition-[width] duration-100 ease-out",
-                  sidebarOpen ? "w-60" : "w-0"
-                )}
-              >
+              <CompanyRail />
+              <ResizableSidebarPane open={sidebarOpen} resizable className="h-full shrink-0">
                 {isInstanceSettingsRoute ? (
                   <InstanceSidebar />
                 ) : isCompanySettingsRoute ? (
                   <CompanySettingsSidebar />
                 ) : (
-                  <Sidebar />
+                  companySidebar
                 )}
-              </div>
+              </ResizableSidebarPane>
             </div>
             <SidebarAccountMenu
               deploymentMode={health?.deploymentMode}