perf(episodic): add idx_events_linked_id to prevent O(N) batch scans

Add `idx_events_linked_id` index to the `events` table in `EpisodicStore` to prevent O(N) full table scans during batch fetch queries filtering by `linked_id` (e.g., `WHERE linked_id IN (...)`). This provides significant speedups during operations such as `count_links_for_semantic_batch`.

Author: sl4m3

src/ledgermind/core/stores/episodic.py

@@ -92,6 +92,10 @@ def _init_db(self):
                 # Performance: Add index for duplicate detection
                 conn.execute("CREATE INDEX IF NOT EXISTS idx_events_duplicate ON events (source, kind, content, timestamp)")
 
+                # Performance: Add index for foreign key-like column to prevent O(N) full table scans
+                # during batch fetch queries like `WHERE linked_id IN (...)`
+                conn.execute("CREATE INDEX IF NOT EXISTS idx_events_linked_id ON events (linked_id)")
+
     def _serialize_context(self, context_data: Any) -> str:
         if not context_data:
             return "{}"