feat: workspace git detection with branch/status badge

When the workspace root is a git repo, a badge in the panel header
shows the current branch name, dirty file count, and ahead/behind
status. Updates on every root directory load.

Backend:
- git_info_for_workspace() in api/workspace.py runs lightweight git
  commands (rev-parse, status --porcelain, rev-list) with 3s timeout
- New GET /api/git-info endpoint returns branch, dirty count, modified,
  untracked, ahead, behind

Frontend:
- _refreshGitBadge() in workspace.js fetches git info on root load
- Git badge element in panel header shows branch + status
- Badge turns gold when workspace has uncommitted changes

Inspired by PR #75 (@MartinNielsenDev).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/index.html

@@ -278,6 +278,7 @@
     <div class="resize-handle" id="rightpanelResize"></div>
     <div class="panel-header">
       <span>Workspace</span>
+      <span class="git-badge" id="gitBadge" style="display:none"></span>
       <div class="panel-actions">
         <button class="panel-icon-btn" id="btnUpDir" title="Parent directory" onclick="navigateUp()" style="display:none">&#8593;</button>
         <button class="panel-icon-btn" id="btnNewFile" title="New file" onclick="promptNewFile()">&#43;</button>