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/validators/plugin.test.ts

@@ -0,0 +1,72 @@
+import { describe, expect, it } from "vitest";
+import { PLUGIN_CAPABILITIES } from "../constants.js";
+import { pluginManagedRoutineDeclarationSchema, pluginUiSlotDeclarationSchema } from "./plugin.js";
+
+describe("plugin capability constants", () => {
+  it("exposes each capability once", () => {
+    expect(new Set(PLUGIN_CAPABILITIES).size).toBe(PLUGIN_CAPABILITIES.length);
+  });
+});
+
+describe("plugin managed routine validators", () => {
+  it("accepts core issue surface visibility values in routine templates", () => {
+    const parsed = pluginManagedRoutineDeclarationSchema.parse({
+      routineKey: "wiki.refresh",
+      title: "Refresh Wiki",
+      issueTemplate: { surfaceVisibility: "default" },
+    });
+
+    expect(parsed.issueTemplate?.surfaceVisibility).toBe("default");
+  });
+
+  it("rejects non-core issue surface visibility values in routine templates", () => {
+    const parsed = pluginManagedRoutineDeclarationSchema.safeParse({
+      routineKey: "wiki.refresh",
+      title: "Refresh Wiki",
+      issueTemplate: { surfaceVisibility: "normal" },
+    });
+
+    expect(parsed.success).toBe(false);
+  });
+});
+
+describe("plugin UI slot validators", () => {
+  it("accepts route-scoped sidebar slots with a routePath", () => {
+    const parsed = pluginUiSlotDeclarationSchema.parse({
+      type: "routeSidebar",
+      id: "wiki-route-sidebar",
+      displayName: "Wiki Sidebar",
+      exportName: "WikiSidebar",
+      routePath: "wiki",
+    });
+
+    expect(parsed.routePath).toBe("wiki");
+  });
+
+  it("requires route-scoped sidebar slots to declare a routePath", () => {
+    const parsed = pluginUiSlotDeclarationSchema.safeParse({
+      type: "routeSidebar",
+      id: "wiki-route-sidebar",
+      displayName: "Wiki Sidebar",
+      exportName: "WikiSidebar",
+    });
+
+    expect(parsed.success).toBe(false);
+    if (parsed.success) return;
+    expect(parsed.error.issues[0]?.message).toBe("routeSidebar slots require routePath");
+  });
+
+  it("keeps reserved company route protection for route-scoped sidebars", () => {
+    const parsed = pluginUiSlotDeclarationSchema.safeParse({
+      type: "routeSidebar",
+      id: "settings-route-sidebar",
+      displayName: "Settings Sidebar",
+      exportName: "SettingsSidebar",
+      routePath: "settings",
+    });
+
+    expect(parsed.success).toBe(false);
+    if (parsed.success) return;
+    expect(parsed.error.issues.some((issue) => issue.message.includes("reserved by the host"))).toBe(true);
+  });
+});