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>

packages/shared/src/constants.ts

@@ -190,6 +190,16 @@ export const ISSUE_ORIGIN_KINDS = [
 export type BuiltInIssueOriginKind = (typeof ISSUE_ORIGIN_KINDS)[number];
 export type PluginIssueOriginKind = `plugin:${string}`;
 export type IssueOriginKind = BuiltInIssueOriginKind | PluginIssueOriginKind;
+export const ISSUE_SURFACE_VISIBILITIES = ["default", "plugin_operation"] as const;
+export type IssueSurfaceVisibility = (typeof ISSUE_SURFACE_VISIBILITIES)[number];
+
+export function pluginOperationIssueOriginKind(pluginKey: string): PluginIssueOriginKind {
+  return `plugin:${pluginKey}:operation`;
+}
+
+export function isPluginOperationIssueOriginKind(originKind: string | null | undefined): boolean {
+  return typeof originKind === "string" && /^plugin:[^:]+:operation(?::|$)/.test(originKind);
+}
 
 export const ISSUE_RELATION_TYPES = ["blocks"] as const;
 export type IssueRelationType = (typeof ISSUE_RELATION_TYPES)[number];
@@ -634,9 +644,12 @@ export const PLUGIN_CAPABILITIES = [
   "issue.comments.create",
   "issue.interactions.create",
   "issue.documents.write",
+  "projects.managed",
+  "routines.managed",
   "agents.pause",
   "agents.resume",
   "agents.invoke",
+  "agents.managed",
   "agent.sessions.create",
   "agent.sessions.list",
   "agent.sessions.send",
@@ -658,6 +671,7 @@ export const PLUGIN_CAPABILITIES = [
   "http.outbound",
   "secrets.read-ref",
   "environment.drivers.register",
+  "local.folders",
   // Agent Tools
   "agent.tools.register",
   // UI
@@ -728,6 +742,7 @@ export const PLUGIN_UI_SLOT_TYPES = [
   "taskDetailView",
   "dashboardWidget",
   "sidebar",
+  "routeSidebar",
   "sidebarPanel",
   "projectSidebarItem",
   "globalToolbarButton",