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>

doc/plugins/PLUGIN_SPEC.md

@@ -27,7 +27,7 @@ Current limitations to keep in mind:
 - Published npm packages are the intended install artifact for deployed plugins.
 - The repo example plugins under `packages/plugins/examples/` are development conveniences. They work from a source checkout and should not be assumed to exist in a generic published build unless they are explicitly shipped with that build.
 - Dynamic plugin install is not yet cloud-ready for horizontally scaled or ephemeral deployments. There is no shared artifact store, install coordination, or cross-node distribution layer yet.
-- The current runtime does not yet ship a real host-provided plugin UI component kit, and it does not support plugin asset uploads/reads. Treat those as future-scope ideas in this spec, not current implementation promises.
+- The current runtime ships a small host-provided plugin UI component kit through `@paperclipai/plugin-sdk/ui`, but does not support plugin asset uploads/reads yet. Treat plugin asset APIs as future-scope ideas, not current implementation promises.
 - Scoped plugin API routes are JSON-only and must be declared in `apiRoutes`.
   They mount under `/api/plugins/:pluginId/api/*`; plugins cannot shadow core
   API routes.
@@ -976,13 +976,23 @@ export function DashboardWidget({ context }: PluginWidgetProps) {
 
 The SDK includes a `ui` subpath export that plugin frontends import. This subpath provides:
 
-- **Bridge hooks**: `usePluginData(key, params)`, `usePluginAction(key)`, `useHostContext()`
+- **Bridge hooks**: `usePluginData(key, params)`, `usePluginAction(key)`, `useHostContext()`, `useHostNavigation()`
 - **Design tokens**: colors, spacing, typography, shadows matching the host theme
 - **Shared components**: `MetricCard`, `StatusBadge`, `DataTable`, `LogView`, `ActionBar`, `Spinner`, etc.
 - **Type definitions**: `PluginPageProps`, `PluginWidgetProps`, `PluginDetailTabProps`
 
 Plugins are encouraged but not required to use the shared components. A plugin may render entirely custom UI as long as it communicates through the bridge.
 
+`useHostNavigation()` is the supported way for plugin UI to navigate to
+Paperclip-internal pages. It exposes `resolveHref(to)`, `navigate(to,
+options?)`, and `linkProps(to, options?)`. Plugin links should prefer
+`linkProps()` so anchors keep real `href` values for copy-link, modifier-click,
+middle-click, and open-in-new-tab behavior while plain left-clicks route through
+the host SPA router. The host resolves company-scoped paths against the active
+company prefix without double-prefixing already-prefixed paths. Plugin UI should
+not use raw same-origin `href`s or `window.location.assign()` for internal
+Paperclip navigation because those can force a full document reload.
+
 ### 19.0.2 Bundle Isolation
 
 Plugin UI bundles are loaded as standard ES modules, not iframed. This gives plugins full rendering performance and access to the host's design tokens.
@@ -1062,6 +1072,11 @@ The host SDK ships shared components that plugins can import to quickly build UI
 | `LogView` | Scrollable log output with timestamps | Webhook deliveries, job output, process logs |
 | `JsonTree` | Collapsible JSON tree for debugging | Raw API responses, plugin state inspection |
 | `Spinner` | Loading indicator | Data fetch states |
+| `FileTree` | Host-styled file/directory tree | Wiki pages, workspace files, import previews |
+| `IssuesList` | Host issue list | Plugin pages that need a native issue view |
+| `AssigneePicker` | Host assignee picker for agents and board users | Creating issues, assigning routines, filtering work |
+| `ProjectPicker` | Host project picker | Creating issues, scoping dashboards, filtering work |
+| `ManagedRoutinesList` | Host routine list | Plugin settings pages that manage routines |
 
 Plugins may also use entirely custom components. The shared components exist to reduce boilerplate and keep visual consistency, not to limit what plugins can render.