Add SessionEnd hook for automatic session summarization

Implements automatic memory extraction when Claude Code sessions end:
- Add SessionEnd hook to hooks.json with 120s timeout
- Create session-end.sh wrapper that parses hook input and runs summarizer
- Create summarize-transcript.py that:
  - Loads transcript from Claude's storage
  - Skips trivial sessions (<3 user messages)
  - Extracts paths/facts via heuristics
  - Uses Claude CLI (subscription auth) for decisions/preferences
  - Saves to memory files with deduplication
  - Updates history index with summarized flag

Uses `claude -p --model haiku` for LLM extraction, leveraging
existing subscription credentials instead of requiring API key.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

hooks/hooks.json

@@ -19,6 +19,17 @@
           }
         ]
       }
+    ],
+    "SessionEnd": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "~/.claude/hooks/scripts/session-end.sh",
+            "timeout": 120
+          }
+        ]
+      }
     ]
   }
 }