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/components/ResizableSidebarPane.test.tsx

@@ -0,0 +1,121 @@
+// @vitest-environment jsdom
+
+import { act } from "react";
+import { createRoot, type Root } from "react-dom/client";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { ResizableSidebarPane } from "./ResizableSidebarPane";
+
+// eslint-disable-next-line @typescript-eslint/no-explicit-any
+(globalThis as any).IS_REACT_ACT_ENVIRONMENT = true;
+
+function pointerEvent(type: string, clientX: number) {
+  const event = new MouseEvent(type, { bubbles: true, clientX });
+  Object.defineProperty(event, "pointerId", { value: 1 });
+  return event;
+}
+
+describe("ResizableSidebarPane", () => {
+  let container: HTMLDivElement;
+  let root: Root;
+
+  beforeEach(() => {
+    window.localStorage.clear();
+    container = document.createElement("div");
+    document.body.appendChild(container);
+    root = createRoot(container);
+  });
+
+  afterEach(() => {
+    act(() => {
+      root.unmount();
+    });
+    container.remove();
+    window.localStorage.clear();
+  });
+
+  function pane() {
+    return container.firstElementChild as HTMLDivElement;
+  }
+
+  function handle() {
+    return container.querySelector('[role="separator"]') as HTMLDivElement | null;
+  }
+
+  it("uses a persisted width when open", () => {
+    window.localStorage.setItem("test.sidebar.width", "320");
+
+    act(() => {
+      root.render(
+        <ResizableSidebarPane open resizable storageKey="test.sidebar.width">
+          <div>Sidebar</div>
+        </ResizableSidebarPane>,
+      );
+    });
+
+    expect(pane().style.width).toBe("320px");
+    expect(handle()?.getAttribute("aria-valuenow")).toBe("320");
+  });
+
+  it("resizes by dragging and persists the new width", () => {
+    act(() => {
+      root.render(
+        <ResizableSidebarPane open resizable storageKey="test.sidebar.width">
+          <div>Sidebar</div>
+        </ResizableSidebarPane>,
+      );
+    });
+
+    const separator = handle();
+    expect(separator).not.toBeNull();
+    separator!.setPointerCapture = vi.fn();
+
+    act(() => {
+      separator!.dispatchEvent(pointerEvent("pointerdown", 240));
+      separator!.dispatchEvent(pointerEvent("pointermove", 320));
+      separator!.dispatchEvent(pointerEvent("pointerup", 320));
+    });
+
+    expect(pane().style.width).toBe("320px");
+    expect(window.localStorage.getItem("test.sidebar.width")).toBe("320");
+  });
+
+  it("supports keyboard resizing and clamps to the configured bounds", () => {
+    act(() => {
+      root.render(
+        <ResizableSidebarPane open resizable storageKey="test.sidebar.width">
+          <div>Sidebar</div>
+        </ResizableSidebarPane>,
+      );
+    });
+
+    const separator = handle();
+    act(() => {
+      separator?.dispatchEvent(new KeyboardEvent("keydown", { key: "ArrowRight", bubbles: true }));
+    });
+    expect(pane().style.width).toBe("256px");
+    expect(window.localStorage.getItem("test.sidebar.width")).toBe("256");
+
+    act(() => {
+      separator?.dispatchEvent(new KeyboardEvent("keydown", { key: "Home", bubbles: true }));
+    });
+    expect(pane().style.width).toBe("208px");
+
+    act(() => {
+      separator?.dispatchEvent(new KeyboardEvent("keydown", { key: "End", bubbles: true }));
+    });
+    expect(pane().style.width).toBe("420px");
+  });
+
+  it("can render without a resize handle", () => {
+    act(() => {
+      root.render(
+        <ResizableSidebarPane open resizable={false}>
+          <div>Sidebar</div>
+        </ResizableSidebarPane>,
+      );
+    });
+
+    expect(handle()).toBeNull();
+    expect(pane().style.width).toBe("240px");
+  });
+});