feat(ui): unify change comparisons and compact message actions

Branch comparisons could retain an old base or omit local edits, while Changes and walkthrough selected their sources independently.

Share branch and commit selectors across both panels, honor exact refs, include local branch edits, and support first-parent commit diffs with the latest 50 commits. Compact message metadata and move touch actions into a shared sheet.

Validated with workspace type-check, lint and build, focused Git and UI tests, and maintainer testing in the app.

packages/web/server/lib/walkthrough/prompt.js

@@ -86,7 +86,9 @@ export function buildPrompt({ digest, fileCount, hunkCount, source, previousWalk
     ? `Uncommitted local changes (${source.scope === 'all' ? 'staged and unstaged' : source.scope}).`
     : source.kind === 'branch'
       ? `All work on branch "${source.headRef}" that is not in "${source.baseRef}". Changes merged in from ${source.baseRef} are already excluded.`
-      : `Pull request #${source.number}.`;
+      : source.kind === 'commit'
+        ? `Only the changes introduced by commit ${source.hash}, relative to its first parent (or the empty tree for a root commit).`
+        : `Pull request #${source.number}.`;
 
   const prompt = `Reviewing: ${sourceLine}