feat: git status bar — show last commit link, remove branch name (#92)

* feat: show last commit hash in status bar, remove branch name

Replace the hardcoded "main" branch display with a clickable short SHA
linking to the GitHub commit. Add Rust get_last_commit_info command that
returns the last commit hash and constructs a GitHub URL from the remote.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* docs: add design/git-status-bar.pen with new status bar layout

Shows the updated status bar with commit hash link, no branch name,
and annotated change callouts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* ci: retrigger — runner 3 pnpm not found (env issue)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/types.ts

@@ -38,6 +38,11 @@ export interface GitCommit {
   date: number // unix timestamp
 }
 
+export interface LastCommitInfo {
+  shortHash: string
+  commitUrl: string | null
+}
+
 export interface ModifiedFile {
   path: string
   relativePath: string