fix: link subagent sessions to parent and hide from session list

Subagent sessions spawned by delegate_task were created with
parent_session_id=NULL and source=cli, making them indistinguishable
from user sessions in hermes sessions list and /resume.

Changes:
- delegate_tool.py: pass parent_agent.session_id to child agent
- run_agent.py: accept parent_session_id param, pass to create_session
- hermes_state.py list_sessions_rich: filter parent_session_id IS NULL
  by default (opt-in include_children=True for callers that need them)
- hermes_state.py delete_session: delete child sessions first (FK)
- hermes_state.py prune_sessions: delete children before parents (FK)

session_search already handles parent_session_id correctly — child
sessions are filtered from recent list and resolved to parent root
in full-text search results.

Fixes #5122

hermes_state.py

@@ -787,6 +787,7 @@ class SessionDB:
         exclude_sources: List[str] = None,
         limit: int = 20,
         offset: int = 0,
+        include_children: bool = False,
     ) -> List[Dict[str, Any]]:
         """List sessions with preview (first user message) and last active timestamp.
 
@@ -795,10 +796,16 @@ class SessionDB:
         last_active (timestamp of last message).
 
         Uses a single query with correlated subqueries instead of N+2 queries.
+
+        By default, child sessions (subagent runs, compression continuations)
+        are excluded.  Pass ``include_children=True`` to include them.
         """
         where_clauses = []
         params = []
 
+        if not include_children:
+            where_clauses.append("s.parent_session_id IS NULL")
+
         if source:
             where_clauses.append("s.source = ?")
             params.append(source)
@@ -1229,22 +1236,38 @@ class SessionDB:
         self._execute_write(_do)
 
     def delete_session(self, session_id: str) -> bool:
-        """Delete a session and all its messages. Returns True if found."""
+        """Delete a session, its child sessions, and all their messages.
+
+        Child sessions (subagent runs, compression continuations) are deleted
+        first to satisfy the ``parent_session_id`` foreign key constraint.
+        Returns True if the session was found and deleted.
+        """
         def _do(conn):
             cursor = conn.execute(
                 "SELECT COUNT(*) FROM sessions WHERE id = ?", (session_id,)
             )
             if cursor.fetchone()[0] == 0:
                 return False
+            # Delete child sessions first (FK constraint)
+            child_ids = [r[0] for r in conn.execute(
+                "SELECT id FROM sessions WHERE parent_session_id = ?",
+                (session_id,),
+            ).fetchall()]
+            for cid in child_ids:
+                conn.execute("DELETE FROM messages WHERE session_id = ?", (cid,))
+                conn.execute("DELETE FROM sessions WHERE id = ?", (cid,))
+            # Delete the session itself
             conn.execute("DELETE FROM messages WHERE session_id = ?", (session_id,))
             conn.execute("DELETE FROM sessions WHERE id = ?", (session_id,))
             return True
         return self._execute_write(_do)
 
     def prune_sessions(self, older_than_days: int = 90, source: str = None) -> int:
-        """
-        Delete sessions older than N days. Returns count of deleted sessions.
-        Only prunes ended sessions (not active ones).
+        """Delete sessions older than N days. Returns count of deleted sessions.
+
+        Only prunes ended sessions (not active ones).  Child sessions whose
+        parents are being pruned are deleted first to satisfy the
+        ``parent_session_id`` foreign key constraint.
         """
         cutoff = time.time() - (older_than_days * 86400)
 
@@ -1260,7 +1283,19 @@ class SessionDB:
                     "SELECT id FROM sessions WHERE started_at < ? AND ended_at IS NOT NULL",
                     (cutoff,),
                 )
-            session_ids = [row["id"] for row in cursor.fetchall()]
+            session_ids = set(row["id"] for row in cursor.fetchall())
+
+            # Delete children first whose parents are in the prune set
+            # (avoids FK constraint errors)
+            for sid in list(session_ids):
+                child_ids = [r[0] for r in conn.execute(
+                    "SELECT id FROM sessions WHERE parent_session_id = ?",
+                    (sid,),
+                ).fetchall()]
+                for cid in child_ids:
+                    conn.execute("DELETE FROM messages WHERE session_id = ?", (cid,))
+                    conn.execute("DELETE FROM sessions WHERE id = ?", (cid,))
+                    session_ids.discard(cid)  # don't double-delete
 
             for sid in session_ids:
                 conn.execute("DELETE FROM messages WHERE session_id = ?", (sid,))