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>

server/src/__tests__/plugin-scoped-api-routes.test.ts

@@ -98,6 +98,7 @@ describe.sequential("plugin scoped API routes", () => {
   const pluginId = "11111111-1111-4111-8111-111111111111";
   const companyId = "22222222-2222-4222-8222-222222222222";
   const agentId = "33333333-3333-4333-8333-333333333333";
+  const peerAgentId = "33333333-3333-4333-8333-333333333334";
   const runId = "44444444-4444-4444-8444-444444444444";
   const issueId = "55555555-5555-4555-8555-555555555555";
 
@@ -252,6 +253,55 @@ describe.sequential("plugin scoped API routes", () => {
     }));
   });
 
+  it("allows non-assignee agents on in-progress required checkout routes without claiming checkout ownership", async () => {
+    const apiRoutes = manifest([
+      {
+        routeKey: "issue.advance",
+        method: "POST",
+        path: "/issues/:issueId/advance",
+        auth: "agent",
+        capability: "api.routes.register",
+        checkoutPolicy: "required-for-agent-in-progress",
+        companyResolution: { from: "issue", param: "issueId" },
+      },
+    ]);
+    mockIssueService.getById.mockResolvedValue({
+      id: issueId,
+      companyId,
+      status: "in_progress",
+      assigneeAgentId: agentId,
+    });
+    const { app, workerManager } = await createApp({
+      actor: {
+        type: "agent",
+        agentId: peerAgentId,
+        companyId,
+        runId,
+        source: "agent_key",
+      },
+      plugin: {
+        id: pluginId,
+        pluginKey: apiRoutes.id,
+        status: "ready",
+        manifestJson: apiRoutes,
+      },
+    });
+
+    const res = await request(app)
+      .post(`/api/plugins/${pluginId}/api/issues/${issueId}/advance`)
+      .send({ step: "next" });
+
+    expect(res.status).toBe(200);
+    expect(mockIssueService.assertCheckoutOwner).not.toHaveBeenCalled();
+    expect(workerManager.call).toHaveBeenCalledWith(pluginId, "handleApiRequest", expect.objectContaining({
+      routeKey: "issue.advance",
+      params: { issueId },
+      body: { step: "next" },
+      actor: expect.objectContaining({ actorType: "agent", agentId: peerAgentId, runId }),
+      companyId,
+    }));
+  });
+
   it("rejects checkout-protected agent routes without a run id before worker dispatch", async () => {
     const apiRoutes = manifest([
       {