[codex] Add workspace diff viewer plugin (#6071)

## Thinking Path

> - Paperclip orchestrates AI agents for zero-human companies.
> - Operators need to inspect what agents changed inside execution and
project workspaces.
> - The existing workspace detail views did not provide a first-party
rich diff surface for staged, unstaged, head, renamed, binary,
oversized, and untracked changes.
> - The plugin system is the intended extension point for optional rich
UI surfaces.
> - This pull request adds a workspace diff plugin plus host services
and shared contracts so Changes tabs can render workspace diffs through
plugin slots.
> - The diff-renderer dependency should stay owned by the plugin package
rather than the core UI app.
> - The dependency surface must stay aligned with repository PR policy,
including intentionally omitting `pnpm-lock.yaml` from the PR.
> - The benefit is a more reviewable workspace surface without
hard-coding the renderer into every page.

## What Changed

- Added `@paperclipai/plugin-workspace-diff`, including diff
normalization, plugin manifest/worker/UI entrypoints, and focused plugin
tests.
- Kept `@pierre/diffs` scoped to `@paperclipai/plugin-workspace-diff`;
removed the core UI lab diff-renderer surface and direct UI package
dependency.
- Added shared workspace diff types and validators, plus plugin SDK
surface for workspace diff host services.
- Added server workspace diff service support and route coverage for
execution/project workspace diff flows.
- Wired Execution Workspace and Project Workspace Changes tabs to load
the diff plugin, including loading/error fallback behavior.
- Added UI tests and fixtures for the Changes tabs and plugin bridge
behavior.
- Added the new plugin package manifest to the Docker deps stage so PR
policy can validate dependency coverage.
- Addressed review hardening around empty untracked patches, workspace
path exposure, project workspace read capability checks, and default
base refs.

## Verification

- `pnpm --filter @paperclipai/plugin-workspace-diff test`
- `pnpm exec vitest run
packages/shared/src/validators/workspace-diff.test.ts
server/src/__tests__/workspace-diff-service.test.ts
ui/src/pages/ProjectWorkspaceDetail.test.tsx
ui/src/pages/ExecutionWorkspaceDetail.test.tsx`
- `pnpm exec vitest run ui/src/plugins/bridge.test.ts
server/src/__tests__/workspace-runtime-routes-authz.test.ts`
- `pnpm --filter @paperclipai/shared typecheck`
- `pnpm --filter @paperclipai/plugin-workspace-diff typecheck`
- `pnpm --filter @paperclipai/server typecheck`
- `pnpm --filter @paperclipai/ui typecheck`
- `node ./scripts/check-docker-deps-stage.mjs`
- Browser screenshot captured from the local worktree dev server:
https://files.catbox.moe/ofdpsp.png
- Confirmed branch is rebased onto `public-gh/master`,
`.github/workflows/pr.yml` is not included in the PR diff,
`ui/package.json` is not included in the PR diff, and `pnpm-lock.yaml`
is not included in the PR diff.

## Risks

- Medium UI integration risk: the Changes tab depends on the plugin slot
and host diff service path.
- Medium dependency risk: this adds `@pierre/diffs` in the plugin
package, but `pnpm-lock.yaml` is intentionally omitted per packaging
instructions because repository automation manages lockfile updates.
- Current CI blocker: downstream frozen installs fail until the
repository policy path for new plugin package dependencies is chosen.
- Diff rendering edge cases are covered for common working-tree and head
diff states, but very large repositories may still expose performance
limits.
- No migrations are included.

> 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 class coding model, tool-enabled local execution
environment. Exact context window 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-orchestration-apis.test.ts

@@ -11,6 +11,7 @@ import {
   companies,
   costEvents,
   createDb,
+  executionWorkspaces,
   heartbeatRuns,
   issueRelations,
   issues,
@@ -67,6 +68,7 @@ describeEmbeddedPostgres("plugin orchestration APIs", () => {
     await db.delete(agentWakeupRequests);
     await db.delete(issueRelations);
     await db.delete(issues);
+    await db.delete(executionWorkspaces);
     await db.delete(pluginManagedResources);
     await db.delete(projects);
     await db.delete(plugins);
@@ -107,6 +109,61 @@ describeEmbeddedPostgres("plugin orchestration APIs", () => {
     return root;
   }
 
+  it("returns plugin-safe execution workspace metadata scoped to the company", async () => {
+    const { companyId } = await seedCompanyAndAgent();
+    const otherCompanyId = randomUUID();
+    const projectId = randomUUID();
+    const workspaceId = randomUUID();
+    await db.insert(companies).values({
+      id: otherCompanyId,
+      name: "Other",
+      issuePrefix: issuePrefix(otherCompanyId),
+      requireBoardApprovalForNewAgents: false,
+    });
+    await db.insert(projects).values({
+      id: projectId,
+      companyId,
+      name: "Workspaces",
+      status: "in_progress",
+    });
+    await db.insert(executionWorkspaces).values({
+      id: workspaceId,
+      companyId,
+      projectId,
+      mode: "isolated_workspace",
+      strategyType: "git_worktree",
+      name: "Feature workspace",
+      status: "active",
+      cwd: "/tmp/paperclip-feature",
+      repoUrl: "https://example.com/paperclip.git",
+      baseRef: "main",
+      branchName: "feature/workspace",
+      providerType: "git_worktree",
+      providerRef: "/tmp/paperclip-feature",
+      metadata: {
+        providerMetadata: { sandboxId: "sandbox-1" },
+        workspaceRealizationRequest: { hiddenInternal: true },
+      },
+    });
+
+    const services = buildHostServices(db, "plugin-record-id", "paperclip.workspace", createEventBusStub());
+
+    await expect(services.executionWorkspaces.get({ workspaceId, companyId })).resolves.toMatchObject({
+      id: workspaceId,
+      companyId,
+      projectId,
+      projectWorkspaceId: null,
+      path: "/tmp/paperclip-feature",
+      cwd: "/tmp/paperclip-feature",
+      repoUrl: "https://example.com/paperclip.git",
+      baseRef: "main",
+      branchName: "feature/workspace",
+      providerType: "git_worktree",
+      providerMetadata: { sandboxId: "sandbox-1" },
+    });
+    await expect(services.executionWorkspaces.get({ workspaceId, companyId: otherCompanyId })).resolves.toBeNull();
+  });
+
   it("creates plugin-origin issues with full orchestration fields and audit activity", async () => {
     const { companyId, agentId } = await seedCompanyAndAgent();
     const blockerIssueId = randomUUID();