feat: self-update checker with one-click update for WebUI + Agent

Shows a blue banner when the webui or hermes-agent git repos are behind
their upstream branches. One-click 'Update Now' button does stash, pull
--ff-only, stash pop, then reloads the page.

Backend (api/updates.py):
- _check_repo(): git fetch + rev-list count with 15s timeout
- check_for_updates(): 30-min server-side cache, thread-safe, skips
  Docker (no .git dir)
- apply_update(): stash (if dirty), pull --ff-only, pop, invalidate cache

Routes:
- GET /api/updates/check -- returns cached {webui, agent} with behind count
- POST /api/updates/apply -- {target: 'webui'|'agent'}

Frontend:
- Blue banner (matches reconnect-banner pattern) with 'Later' / 'Update Now'
- Non-blocking boot check via fire-and-forget .then(), once per tab session
- sessionStorage guards prevent re-checking and re-showing after dismiss

Settings:
- 'Check for updates' checkbox (default: on) -- when off, no git operations
- Removed 'Default Workspace' dropdown to keep settings panel compact

Performance:
- Server cache: git fetch at most 2x/hour regardless of client count
- sessionStorage: one check per browser tab session
- _check_in_progress flag prevents concurrent fetch storms
- Fire-and-forget: does NOT block the boot sequence

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

api/updates.py

@@ -0,0 +1,153 @@
+"""
+Hermes Web UI -- Self-update checker.
+
+Checks if the webui and hermes-agent git repos are behind their upstream
+branches. Results are cached server-side (30-min TTL) so git fetch runs
+at most twice per hour regardless of client count.
+
+Skips repos that are not git checkouts (e.g. Docker baked images where
+.git does not exist).
+"""
+import subprocess
+import threading
+import time
+from pathlib import Path
+
+from api.config import REPO_ROOT
+
+# Lazy -- may be None if agent not found
+try:
+    from api.config import _AGENT_DIR
+except ImportError:
+    _AGENT_DIR = None
+
+_update_cache = {'webui': None, 'agent': None, 'checked_at': 0}
+_cache_lock = threading.Lock()
+_check_in_progress = False
+CACHE_TTL = 1800  # 30 minutes
+
+
+def _run_git(args, cwd, timeout=10):
+    """Run a git command and return (stdout, ok)."""
+    try:
+        r = subprocess.run(
+            ['git'] + args, cwd=str(cwd), capture_output=True,
+            text=True, timeout=timeout,
+        )
+        return r.stdout.strip(), r.returncode == 0
+    except (subprocess.TimeoutExpired, FileNotFoundError, OSError):
+        return '', False
+
+
+def _detect_default_branch(path):
+    """Detect the remote default branch (master or main)."""
+    out, ok = _run_git(['symbolic-ref', 'refs/remotes/origin/HEAD'], path)
+    if ok and out:
+        # refs/remotes/origin/master -> master
+        return out.split('/')[-1]
+    # Fallback: try master, then main
+    for branch in ('master', 'main'):
+        _, ok = _run_git(['rev-parse', '--verify', f'origin/{branch}'], path)
+        if ok:
+            return branch
+    return 'master'
+
+
+def _check_repo(path, name):
+    """Check if a git repo is behind its upstream. Returns dict or None."""
+    if path is None or not (path / '.git').exists():
+        return None
+
+    # Fetch latest from origin (network call, cached by TTL)
+    _, fetch_ok = _run_git(['fetch', 'origin', '--quiet'], path, timeout=15)
+    if not fetch_ok:
+        return {'name': name, 'behind': 0, 'error': 'fetch failed'}
+
+    branch = _detect_default_branch(path)
+
+    # Count commits behind
+    out, ok = _run_git(['rev-list', '--count', f'HEAD..origin/{branch}'], path)
+    behind = int(out) if ok and out.isdigit() else 0
+
+    # Get short SHAs for display
+    current, _ = _run_git(['rev-parse', '--short', 'HEAD'], path)
+    latest, _ = _run_git(['rev-parse', '--short', f'origin/{branch}'], path)
+
+    return {
+        'name': name,
+        'behind': behind,
+        'current_sha': current,
+        'latest_sha': latest,
+        'branch': branch,
+    }
+
+
+def check_for_updates(force=False):
+    """Return cached update status for webui and agent repos."""
+    global _check_in_progress
+    with _cache_lock:
+        if not force and time.time() - _update_cache['checked_at'] < CACHE_TTL:
+            return dict(_update_cache)
+        if _check_in_progress:
+            return dict(_update_cache)  # another thread is already checking
+        _check_in_progress = True
+
+    try:
+        # Run checks outside the lock (network I/O)
+        webui_info = _check_repo(REPO_ROOT, 'webui')
+        agent_info = _check_repo(_AGENT_DIR, 'agent')
+
+        with _cache_lock:
+            _update_cache['webui'] = webui_info
+            _update_cache['agent'] = agent_info
+            _update_cache['checked_at'] = time.time()
+            return dict(_update_cache)
+    finally:
+        _check_in_progress = False
+
+
+def apply_update(target):
+    """Stash, pull --ff-only, pop for the given target repo."""
+    if target == 'webui':
+        path = REPO_ROOT
+    elif target == 'agent':
+        path = _AGENT_DIR
+    else:
+        return {'ok': False, 'message': f'Unknown target: {target}'}
+
+    if path is None or not (path / '.git').exists():
+        return {'ok': False, 'message': 'Not a git repository'}
+
+    branch = _detect_default_branch(path)
+
+    # Check for dirty working tree
+    status_out, _ = _run_git(['status', '--porcelain'], path)
+    stashed = False
+    if status_out:
+        _, ok = _run_git(['stash'], path)
+        if not ok:
+            return {'ok': False, 'message': 'Failed to stash local changes'}
+        stashed = True
+
+    # Pull with ff-only (no merge commits)
+    pull_out, pull_ok = _run_git(['pull', '--ff-only', 'origin', branch], path, timeout=30)
+    if not pull_ok:
+        if stashed:
+            _run_git(['stash', 'pop'], path)
+        return {'ok': False, 'message': f'Pull failed: {pull_out[:200]}'}
+
+    # Pop stash if we stashed
+    if stashed:
+        _, pop_ok = _run_git(['stash', 'pop'], path)
+        if not pop_ok:
+            return {
+                'ok': False,
+                'message': 'Updated but stash pop failed -- manual merge needed',
+                'stash_conflict': True,
+            }
+
+    # Invalidate cache
+    with _cache_lock:
+        _update_cache['checked_at'] = 0
+
+    return {'ok': True, 'message': f'{target} updated successfully', 'target': target}