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-routes-authz.test.ts

@@ -6,6 +6,8 @@ const mockRegistry = vi.hoisted(() => ({
   getById: vi.fn(),
   getByKey: vi.fn(),
   upsertConfig: vi.fn(),
+  getCompanySettings: vi.fn(),
+  upsertCompanySettings: vi.fn(),
 }));
 
 const mockLifecycle = vi.hoisted(() => ({
@@ -317,6 +319,61 @@ describe.sequential("scoped plugin API routes", () => {
   }, 20_000);
 });
 
+describe.sequential("plugin local folder routes", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    mockRegistry.getCompanySettings.mockResolvedValue(null);
+  });
+
+  function readyLocalFolderPlugin() {
+    mockRegistry.getById.mockResolvedValue({
+      id: pluginId,
+      pluginKey: "paperclip.example",
+      version: "1.0.0",
+      status: "ready",
+      manifestJson: {
+        id: "paperclip.example",
+        capabilities: ["local.folders"],
+        localFolders: [
+          {
+            folderKey: "content-root",
+            displayName: "Content root",
+            access: "readWrite",
+            requiredDirectories: ["docs"],
+            requiredFiles: ["README.md"],
+          },
+        ],
+      },
+    });
+  }
+
+  it("rejects validation for undeclared local folder keys", async () => {
+    readyLocalFolderPlugin();
+    const { app } = await createApp(boardActor());
+
+    const res = await request(app)
+      .post(`/api/plugins/${pluginId}/companies/${companyA}/local-folders/ssh/validate`)
+      .send({ path: "/tmp" });
+
+    expect(res.status).toBe(400);
+    expect(res.body.error).toContain("Local folder key is not declared");
+    expect(mockRegistry.upsertCompanySettings).not.toHaveBeenCalled();
+  });
+
+  it("rejects saving undeclared local folder keys", async () => {
+    readyLocalFolderPlugin();
+    const { app } = await createApp(boardActor());
+
+    const res = await request(app)
+      .put(`/api/plugins/${pluginId}/companies/${companyA}/local-folders/ssh`)
+      .send({ path: "/tmp" });
+
+    expect(res.status).toBe(400);
+    expect(res.body.error).toContain("Local folder key is not declared");
+    expect(mockRegistry.upsertCompanySettings).not.toHaveBeenCalled();
+  });
+});
+
 describe.sequential("plugin tool and bridge authz", () => {
   beforeEach(() => {
     vi.clearAllMocks();