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/plugins/slots.tsx

@@ -63,6 +63,33 @@ export type ResolvedPluginSlot = PluginUiSlotDeclaration & {
   pluginVersion: string;
 };
 
+/**
+ * Returns the unique `routeSidebar` slot that pairs with a single `page` slot
+ * for the given route, or `null` if no unambiguous pairing exists.
+ *
+ * Used to detect when a route is taken over by a plugin's full-page sidebar so
+ * host chrome (breadcrumb, in-page Back) can be suppressed.
+ */
+export function resolveRouteSidebarSlot(
+  slots: ResolvedPluginSlot[],
+  routePath: string | null,
+): ResolvedPluginSlot | null {
+  if (!routePath) return null;
+
+  const pageMatches = slots.filter((slot) => slot.type === "page" && slot.routePath === routePath);
+  if (pageMatches.length !== 1) return null;
+
+  const [pageSlot] = pageMatches;
+  const sidebarMatches = slots.filter((slot) =>
+    slot.type === "routeSidebar"
+    && slot.routePath === routePath
+    && slot.pluginId === pageSlot.pluginId,
+  );
+
+  if (sidebarMatches.length !== 1) return null;
+  return sidebarMatches[0] ?? null;
+}
+
 type PluginSlotComponentProps = {
   slot: ResolvedPluginSlot;
   context: PluginSlotContext;
@@ -257,8 +284,30 @@ function getShimBlobUrl(specifier: "react" | "react-dom" | "react-dom/client" |
     case "sdk-ui":
       source = `
         const SDK = globalThis.__paperclipPluginBridge__?.sdkUi ?? {};
-        const { usePluginData, usePluginAction, useHostContext, usePluginStream, usePluginToast } = SDK;
-        export { usePluginData, usePluginAction, useHostContext, usePluginStream, usePluginToast };
+        function missing(name) {
+          return function MissingPaperclipSdkUiComponent() {
+            throw new Error('Paperclip plugin UI runtime is not initialized for "' + name + '". Ensure the host loaded the plugin bridge before rendering this UI module.');
+          };
+        }
+        const { usePluginData, usePluginAction, useHostContext, useHostLocation, useHostNavigation, usePluginStream, usePluginToast } = SDK;
+        const MetricCard = SDK.MetricCard ?? missing("MetricCard");
+        const StatusBadge = SDK.StatusBadge ?? missing("StatusBadge");
+        const DataTable = SDK.DataTable ?? missing("DataTable");
+        const TimeseriesChart = SDK.TimeseriesChart ?? missing("TimeseriesChart");
+        const MarkdownBlock = SDK.MarkdownBlock ?? missing("MarkdownBlock");
+        const MarkdownEditor = SDK.MarkdownEditor ?? missing("MarkdownEditor");
+        const KeyValueList = SDK.KeyValueList ?? missing("KeyValueList");
+        const ActionBar = SDK.ActionBar ?? missing("ActionBar");
+        const LogView = SDK.LogView ?? missing("LogView");
+        const JsonTree = SDK.JsonTree ?? missing("JsonTree");
+        const Spinner = SDK.Spinner ?? missing("Spinner");
+        const ErrorBoundary = SDK.ErrorBoundary ?? missing("ErrorBoundary");
+        const FileTree = SDK.FileTree ?? missing("FileTree");
+        const IssuesList = SDK.IssuesList ?? missing("IssuesList");
+        const AssigneePicker = SDK.AssigneePicker ?? missing("AssigneePicker");
+        const ProjectPicker = SDK.ProjectPicker ?? missing("ProjectPicker");
+        const ManagedRoutinesList = SDK.ManagedRoutinesList ?? missing("ManagedRoutinesList");
+        export { usePluginData, usePluginAction, useHostContext, useHostLocation, useHostNavigation, usePluginStream, usePluginToast, MetricCard, StatusBadge, DataTable, TimeseriesChart, MarkdownBlock, MarkdownEditor, KeyValueList, ActionBar, LogView, JsonTree, Spinner, ErrorBoundary, FileTree, IssuesList, AssigneePicker, ProjectPicker, ManagedRoutinesList };
       `;
       break;
   }