[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/src/worker.ts

@@ -1,4 +1,4 @@
-import { definePlugin, runWorker } from "@paperclipai/plugin-sdk";
+import { definePlugin, runWorker, type PluginContext } from "@paperclipai/plugin-sdk";
 import { workspaceDiffQuerySchema } from "./contracts.js";
 import { workspaceDiffService } from "./workspace-diff.js";
 
@@ -23,6 +23,25 @@ export function resolveDefaultBaseRef(input: {
     ?? readOptionalString(input.projectWorkspaceRepoRef);
 }
 
+async function resolveProjectWorkspaceDefaultBaseRef(input: {
+  ctx: PluginContext;
+  projectId: string;
+  companyId: string;
+  projectWorkspaceId?: string | null;
+}): Promise<string | null> {
+  if (!input.projectId) return null;
+  const workspaces = await input.ctx.projects.listWorkspaces(input.projectId, input.companyId);
+  const projectWorkspace = input.projectWorkspaceId
+    ? workspaces.find((candidate) => candidate.id === input.projectWorkspaceId)
+    : workspaces.find((candidate) => candidate.isPrimary) ?? workspaces[0] ?? null;
+  return projectWorkspace
+    ? resolveDefaultBaseRef({
+      projectWorkspaceDefaultRef: projectWorkspace.defaultRef,
+      projectWorkspaceRepoRef: projectWorkspace.repoRef,
+    })
+    : null;
+}
+
 const plugin = definePlugin({
   async setup(ctx) {
     ctx.logger.info(`${PLUGIN_NAME} plugin setup`);
@@ -61,15 +80,13 @@ const plugin = definePlugin({
         throw new Error("Workspace not found");
       }
       let projectWorkspaceDefaultBaseRef: string | null = null;
-      if (!readOptionalString(workspace.baseRef) && workspace.projectWorkspaceId) {
-        const workspaces = await ctx.projects.listWorkspaces(workspace.projectId, companyId);
-        const projectWorkspace = workspaces.find((candidate) => candidate.id === workspace.projectWorkspaceId);
-        projectWorkspaceDefaultBaseRef = projectWorkspace
-          ? resolveDefaultBaseRef({
-            projectWorkspaceDefaultRef: projectWorkspace.defaultRef,
-            projectWorkspaceRepoRef: projectWorkspace.repoRef,
-          })
-          : null;
+      if (!readOptionalString(workspace.baseRef)) {
+        projectWorkspaceDefaultBaseRef = await resolveProjectWorkspaceDefaultBaseRef({
+          ctx,
+          projectId: workspace.projectId || readString(params.projectId),
+          companyId,
+          projectWorkspaceId: workspace.projectWorkspaceId,
+        });
       }
 
       return workspaceDiff.getDiff({