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/api/plugins.test.ts

@@ -0,0 +1,64 @@
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const mockApi = vi.hoisted(() => ({
+  get: vi.fn(),
+  post: vi.fn(),
+  put: vi.fn(),
+}));
+
+vi.mock("./client", () => ({
+  api: mockApi,
+}));
+
+import { pluginsApi } from "./plugins";
+
+describe("pluginsApi local folders", () => {
+  beforeEach(() => {
+    mockApi.get.mockReset();
+    mockApi.post.mockReset();
+    mockApi.put.mockReset();
+    mockApi.get.mockResolvedValue({});
+    mockApi.post.mockResolvedValue({});
+    mockApi.put.mockResolvedValue({});
+  });
+
+  it("lists company-scoped local folders for a plugin", async () => {
+    await pluginsApi.listLocalFolders("plugin-1", "company-1");
+
+    expect(mockApi.get).toHaveBeenCalledWith(
+      "/plugins/plugin-1/companies/company-1/local-folders",
+    );
+  });
+
+  it("validates a candidate folder path without saving", async () => {
+    await pluginsApi.validateLocalFolder("plugin-1", "company-1", "wiki-root", {
+      path: "/tmp/wiki",
+      access: "readWrite",
+      requiredFiles: ["WIKI.md"],
+    });
+
+    expect(mockApi.post).toHaveBeenCalledWith(
+      "/plugins/plugin-1/companies/company-1/local-folders/wiki-root/validate",
+      {
+        path: "/tmp/wiki",
+        access: "readWrite",
+        requiredFiles: ["WIKI.md"],
+      },
+    );
+  });
+
+  it("saves through the local-folder PUT endpoint", async () => {
+    await pluginsApi.configureLocalFolder("plugin-1", "company-1", "wiki-root", {
+      path: "/tmp/wiki",
+      requiredDirectories: ["wiki"],
+    });
+
+    expect(mockApi.put).toHaveBeenCalledWith(
+      "/plugins/plugin-1/companies/company-1/local-folders/wiki-root",
+      {
+        path: "/tmp/wiki",
+        requiredDirectories: ["wiki"],
+      },
+    );
+  });
+});