fix(cli): non-blocking startup update check and banner deduplication

- Add background thread mechanism (prefetch_update_check/get_update_result)
  so git fetch runs in parallel with skill sync and agent init
- Fix repo path fallback in check_for_updates() for dev installs
- Remove duplicate build_welcome_banner (~180 lines) and
  _format_context_length from cli.py — the banner.py version is
  now the single source of truth
- Port skin banner_hero/banner_logo support and terminal width check
  from cli.py's version into banner.py
- Add update status output to hermes version command
- Add unit tests for update check, prefetch, and version string

hermes_cli/main.py

@@ -480,6 +480,13 @@ def cmd_chat(args):
         print("You can run 'hermes setup' at any time to configure.")
         sys.exit(1)
 
+    # Start update check in background (runs while other init happens)
+    try:
+        from hermes_cli.banner import prefetch_update_check
+        prefetch_update_check()
+    except Exception:
+        pass
+
     # Sync bundled skills on every CLI launch (fast -- skips unchanged skills)
     try:
         from tools.skills_sync import sync_skills
@@ -1863,6 +1870,18 @@ def cmd_version(args):
     except ImportError:
         print("OpenAI SDK: Not installed")
 
+    # Show update status (synchronous — acceptable since user asked for version info)
+    try:
+        from hermes_cli.banner import check_for_updates
+        behind = check_for_updates()
+        if behind and behind > 0:
+            commits_word = "commit" if behind == 1 else "commits"
+            print(f"Update available: {behind} {commits_word} behind — run 'hermes update'")
+        elif behind == 0:
+            print("Up to date")
+    except Exception:
+        pass
+
 
 def cmd_uninstall(args):
     """Uninstall Hermes Agent."""