feat(hooks): add UserPromptSubmit hook handler

- user_prompt_handler.py: Full handler with signal detection pipeline
  - AUTO capture for high-confidence signals (≥0.95)
  - SUGGEST for medium-confidence (0.7-0.95)
  - XML-formatted suggestions for additionalContext
  - Non-blocking error handling
- hooks/user_prompt.py: Wrapper script delegating to handler

Author: zircote

hooks/user_prompt.py

@@ -0,0 +1,50 @@
+#!/usr/bin/env python3
+"""Hook: Detect memorable content in user prompts.
+
+This hook detects signals indicating memorable content (decisions, learnings,
+blockers, etc.) in user prompts and either captures automatically or suggests
+capture to the user.
+
+The hook uses pattern-based signal detection combined with novelty checking
+to avoid duplicate captures.
+
+Environment Variables:
+    HOOK_ENABLED: Master switch for hooks (default: true)
+    HOOK_USER_PROMPT_ENABLED: Enable this hook (default: true)
+    HOOK_DEBUG: Enable debug logging (default: false)
+
+Exit codes:
+    0 - Success (non-blocking)
+"""
+
+from __future__ import annotations
+
+import sys
+
+
+def main() -> None:
+    """Main hook entry point.
+
+    Delegates to the user_prompt_handler module for actual processing.
+    Falls back gracefully if the library is not installed.
+    """
+    try:
+        from git_notes_memory.hooks.user_prompt_handler import main as handler_main
+
+        handler_main()
+    except ImportError:
+        # Library not installed, exit silently (non-blocking)
+        import json
+
+        print(json.dumps({"continue": True}))
+        sys.exit(0)
+    except Exception:
+        # Any unexpected error, exit silently (non-blocking)
+        import json
+
+        print(json.dumps({"continue": True}))
+        sys.exit(0)
+
+
+if __name__ == "__main__":
+    main()