Scope git history to current diagram by default

- History panel defaults to "This diagram" / "All" toggle
- "This diagram" filters git log to commits that touched only that
  diagram's JSON file (git log -- diagrams/0001_*.json)
- Automatically switches to "All" when no diagram is open
- Restore flow still works per-commit, per-file

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

frontend/index.html

@@ -449,8 +449,14 @@
 
       <!-- History section -->
       <div class="git-section" style="flex:1">
-        <label class="git-section-title">Commit history</label>
-        <div id="git-log-list" style="max-height:320px;overflow-y:auto;font-size:11px;display:flex;flex-direction:column;gap:1px"></div>
+        <div style="display:flex;align-items:center;justify-content:space-between">
+          <label class="git-section-title" id="git-log-title">Commit history</label>
+          <div style="display:flex;gap:2px">
+            <button id="btn-git-log-diagram" class="git-scope-btn active" title="Show commits for current diagram only">This diagram</button>
+            <button id="btn-git-log-all"     class="git-scope-btn"        title="Show all commits">All</button>
+          </div>
+        </div>
+        <div id="git-log-list" style="max-height:300px;overflow-y:auto;font-size:11px;display:flex;flex-direction:column;gap:1px"></div>
       </div>
 
     </div>