fix: gists show the conversation's real date, not the summarize-run date

Summaries displayed s.created_at (set to now() at summarize time), so every
imported gist read 2026-06-16. Derive the actual session date from the earliest
exchange timestamp (MIN(created_at) per session — the preserved original date,
same source the era rollups use) via a correlated subquery in the summary
readers. New Summary.session_started_at field; chat shows it (falling back to
created_at). No schema change / backfill needed — always correct from source.

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

lyra/memory.py

@@ -122,7 +122,8 @@ class Summary:
     session_id: str
     content: str
     last_exchange_id: int
-    created_at: str
+    created_at: str  # when the gist was generated
+    session_started_at: str | None = None  # when the conversation actually happened
     score: float | None = None
 
 
@@ -313,8 +314,9 @@ def store_summary(session_id: str, content: str, last_exchange_id: int) -> None:
 def get_summary(session_id: str) -> Summary | None:
     conn = _connection()
     r = conn.execute(
-        "SELECT session_id, content, last_exchange_id, created_at FROM summaries "
-        "WHERE session_id = ?",
+        "SELECT session_id, content, last_exchange_id, created_at, "
+        "(SELECT MIN(e.created_at) FROM exchanges e WHERE e.session_id = summaries.session_id) "
+        "AS started_at FROM summaries WHERE session_id = ?",
         (session_id,),
     ).fetchone()
     if r is None:
@@ -324,6 +326,7 @@ def get_summary(session_id: str) -> Summary | None:
         content=r["content"],
         last_exchange_id=r["last_exchange_id"],
         created_at=r["created_at"],
+        session_started_at=r["started_at"],
     )
 
 
@@ -343,8 +346,9 @@ def list_summaries() -> list[Summary]:
     """Every session gist (for the profile/era consolidation passes)."""
     conn = _connection()
     rows = conn.execute(
-        "SELECT session_id, content, last_exchange_id, created_at FROM summaries "
-        "ORDER BY created_at ASC"
+        "SELECT session_id, content, last_exchange_id, created_at, "
+        "(SELECT MIN(e.created_at) FROM exchanges e WHERE e.session_id = summaries.session_id) "
+        "AS started_at FROM summaries ORDER BY started_at ASC"
     ).fetchall()
     return [
         Summary(
@@ -352,6 +356,7 @@ def list_summaries() -> list[Summary]:
             content=r["content"],
             last_exchange_id=r["last_exchange_id"],
             created_at=r["created_at"],
+            session_started_at=r["started_at"],
         )
         for r in rows
     ]
@@ -560,7 +565,11 @@ def recall_summaries(query: str, k: int = 3, exclude_session: str | None = None)
     q = np.asarray(q_vec, dtype=np.float32)
 
     conn = _connection()
-    sql = "SELECT session_id, content, embedding, last_exchange_id, created_at FROM summaries"
+    sql = (
+        "SELECT session_id, content, embedding, last_exchange_id, created_at, "
+        "(SELECT MIN(e.created_at) FROM exchanges e WHERE e.session_id = summaries.session_id) "
+        "AS started_at FROM summaries"
+    )
     params: tuple = ()
     if exclude_session is not None:
         sql += " WHERE session_id != ?"
@@ -580,6 +589,7 @@ def recall_summaries(query: str, k: int = 3, exclude_session: str | None = None)
             content=rows[i]["content"],
             last_exchange_id=rows[i]["last_exchange_id"],
             created_at=rows[i]["created_at"],
+            session_started_at=rows[i]["started_at"],
             score=float(scores[i]),
         )
         for i in top_idx