feat: ChatGPT chat-log importer

Import the parser's {title, messages} JSON into Lyra's memory so past
conversations seed recall (and, later, the era-rollup tier).

- lyra/ingest.py: one conversation -> one session, text messages -> exchanges;
  skips non-text (image asset) messages and non user/assistant roles; embeddings
  batched; idempotent by filename-derived session id; `lyra-import <dir>` CLI
- memory.add_exchanges_bulk: batched insert of pre-embedded rows

Format has no timestamps yet, so imports are stamped at import time; a future
dated export will let era memory group by real calendar time.

Verified on the 68-file lyra dev set: 7519 exchanges, idempotent re-run, recall
returns relevant history.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lyra/memory.py

@@ -108,6 +108,22 @@ def remember(session_id: str, role: str, content: str) -> int:
     return int(cur.lastrowid)
 
 
+def add_exchanges_bulk(session_id: str, rows: list[tuple[str, str, list[float], str]]) -> int:
+    """Insert many pre-embedded exchanges at once.
+
+    Each row is (role, content, embedding, created_at). Used by the importer to
+    avoid one INSERT (and one embed round-trip) per message. Returns row count.
+    """
+    conn = _connection()
+    with conn:
+        conn.executemany(
+            "INSERT INTO exchanges (session_id, role, content, embedding, created_at) "
+            "VALUES (?, ?, ?, ?, ?)",
+            [(session_id, role, content, _to_blob(emb), ca) for role, content, emb, ca in rows],
+        )
+    return len(rows)
+
+
 def recent(session_id: str, n: int = 10) -> list[Exchange]:
     """Last `n` exchanges from a session, oldest first."""
     conn = _connection()