[codex] Workspace diff polish (#6383)

## Thinking Path

> - Paperclip gives operators a workspace diff plugin so they can
inspect agent changes before review
> - The diff view needs reliable base-ref defaults and controls that
stay usable while scrolling large diffs
> - The working branch mixed those plugin improvements with unrelated
server and cloud work
> - Keeping the workspace diff plugin changes isolated makes them easy
to test and review
> - This pull request polishes the workspace diff plugin controls,
base-ref behavior, and sticky headers
> - The benefit is a more predictable diff review surface for agent
workspaces

## What Changed

- Fixed workspace diff default base-ref resolution.
- Improved split/unified and working-tree/against-ref pane controls.
- Made workspace diff headers stay sticky while scrolling.
- Added a review screenshot at
`screenshots/PAP-9841-workspace-diff.png`.

## Verification

- `pnpm install --frozen-lockfile --ignore-scripts`
- `pnpm --filter @paperclipai/plugin-sdk build`
- `pnpm --filter @paperclipai/plugin-workspace-diff exec vitest run
tests/plugin.spec.ts`
- Result: 9 tests passed.

## Risks

- UI-only plugin branch with low data risk.
- The default base-ref inference should be reviewed against unusual
worktree/upstream combinations.

> 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-based coding agent with local shell/git/tool use.
Exact hosted model ID and context-window size are not exposed by the
local Paperclip adapter 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/plugins/plugin-workspace-diff/tests/plugin.spec.ts

@@ -227,6 +227,115 @@ describe("workspace diff plugin", () => {
     });
   });
 
+  it("uses the primary project workspace default ref when execution workspace has no workspace link", async () => {
+    const root = await createGitWorkspace();
+    await git(root, ["checkout", "-b", "feature"]);
+    await fs.writeFile(path.join(root, "src/app.ts"), "export const value = 5;\n");
+    await git(root, ["add", "src/app.ts"]);
+    await git(root, ["commit", "-m", "feature change"]);
+    const harness = createTestHarness({ manifest });
+    harness.seed({
+      executionWorkspaces: [{
+        id: "workspace-1",
+        companyId: "company-1",
+        projectId: "project-1",
+        projectWorkspaceId: null,
+        path: root,
+        cwd: root,
+        repoUrl: null,
+        baseRef: null,
+        branchName: "feature",
+        providerType: "git_worktree",
+        providerMetadata: null,
+      }],
+    });
+    harness.ctx.projects.listWorkspaces = async (projectId, companyId) => {
+      expect(projectId).toBe("project-1");
+      expect(companyId).toBe("company-1");
+      return [{
+        id: "project-workspace-1",
+        projectId: "project-1",
+        name: "Primary",
+        path: root,
+        repoUrl: null,
+        repoRef: "feature",
+        defaultRef: "main",
+        isPrimary: true,
+        createdAt: new Date().toISOString(),
+        updatedAt: new Date().toISOString(),
+      }];
+    };
+    await plugin.definition.setup(harness.ctx);
+
+    const result = await harness.getData("workspace-diff", {
+      workspaceId: "workspace-1",
+      companyId: "company-1",
+      projectId: "project-1",
+      view: "head",
+      baseRef: null,
+      includeUntracked: false,
+    });
+
+    expect(result).toMatchObject({
+      baseRef: "main",
+      defaultBaseRef: "main",
+      stats: { fileCount: 1 },
+      files: [expect.objectContaining({ path: "src/app.ts" })],
+    });
+  });
+
+  it("infers the default base ref from the execution workspace branch upstream", async () => {
+    const root = await createGitWorkspace();
+    await git(root, ["update-ref", "refs/remotes/origin/master", "HEAD"]);
+    await git(root, ["checkout", "-b", "feature"]);
+    await git(root, ["config", "branch.feature.remote", "origin"]);
+    await git(root, ["config", "branch.feature.merge", "refs/heads/master"]);
+    await fs.writeFile(path.join(root, "src/app.ts"), "export const value = 6;\n");
+    await git(root, ["add", "src/app.ts"]);
+    await git(root, ["commit", "-m", "feature change"]);
+    const harness = createTestHarness({ manifest });
+    harness.seed({
+      executionWorkspaces: [{
+        id: "workspace-1",
+        companyId: "company-1",
+        projectId: "project-1",
+        projectWorkspaceId: null,
+        path: root,
+        cwd: root,
+        repoUrl: null,
+        baseRef: null,
+        branchName: "feature",
+        providerType: "git_worktree",
+        providerMetadata: null,
+      }],
+    });
+    await plugin.definition.setup(harness.ctx);
+
+    await expect(harness.getData("workspace-diff", {
+      workspaceId: "workspace-1",
+      companyId: "company-1",
+      view: "working-tree",
+      includeUntracked: false,
+    })).resolves.toMatchObject({
+      baseRef: null,
+      defaultBaseRef: "origin/master",
+      stats: { fileCount: 0 },
+    });
+
+    await expect(harness.getData("workspace-diff", {
+      workspaceId: "workspace-1",
+      companyId: "company-1",
+      view: "head",
+      baseRef: null,
+      includeUntracked: false,
+    })).resolves.toMatchObject({
+      baseRef: "origin/master",
+      defaultBaseRef: "origin/master",
+      stats: { fileCount: 1 },
+      files: [expect.objectContaining({ path: "src/app.ts" })],
+    });
+  });
+
   it("returns a clear bridge error when required context is missing", async () => {
     const harness = createTestHarness({ manifest });
     await plugin.definition.setup(harness.ctx);