perf: Add tool_id index for 77x faster error queries

get_error_details() was taking 25s on 160K events due to self-join
on tool_id without an index. Migration v7 adds idx_events_tool_id,
reducing query time to 0.3s.

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

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

Author: evansenter

src/session_analytics/storage.py

@@ -166,7 +166,7 @@ class BusEvent:
 DEFAULT_DB_PATH = Path.home() / ".claude" / "contrib" / "analytics" / "data.db"
 
 # Schema version for migrations
-SCHEMA_VERSION = 6
+SCHEMA_VERSION = 7
 
 # Migration functions: dict of version -> (migration_name, migration_func)
 # Each migration upgrades FROM version-1 TO version
@@ -379,6 +379,17 @@ def migrate_v6(conn):
     conn.execute("CREATE INDEX IF NOT EXISTS idx_bus_events_repo ON bus_events(repo)")
 
 
+@migration(7, "add_tool_id_index")
+def migrate_v7(conn):
+    """Add index on tool_id for faster self-joins.
+
+    The query_error_details() function joins events to itself on tool_id
+    to correlate tool_result errors with their tool_use parameters.
+    Without this index, queries took ~25s on 160K events.
+    """
+    conn.execute("CREATE INDEX IF NOT EXISTS idx_events_tool_id ON events(tool_id)")
+
+
 class SQLiteStorage:
     """SQLite-backed storage for session analytics."""