fix: signal compression boundary to context engine

When _compress_context rotates session_id (compression split), fire
on_session_start(new_sid, boundary_reason="compression",
old_session_id=<old>) on the active context engine. Plugin engines
(e.g. hermes-lcm) use this to preserve DAG lineage across the rollover
instead of re-initializing fresh per-session state.

Built-in ContextCompressor.on_session_start accepts **kwargs and ignores
them — no behavior change for default users.

Closes hermes-lcm#68 symptom: after Hermes compressed and minted a new
physical session, LCM was treating the split as a fresh /new and losing
continuity (compression_count: 1, store_messages: 0, dag_nodes: 0).

Credit: @Tosko4 (PR #13370) — minimized scope to the boundary_reason
signal only; the broader session-lifecycle refactor will be taken in
separate PRs if justified by concrete plugin need.

run_agent.py

@@ -8160,6 +8160,22 @@ class AIAgent:
             except Exception as e:
                 logger.warning("Session DB compression split failed — new session will NOT be indexed: %s", e)
 
+        # Notify the context engine that the session_id rotated because of
+        # compression (not a fresh /new). Plugin engines (e.g. hermes-lcm) use
+        # boundary_reason="compression" to preserve DAG lineage across the
+        # rollover instead of re-initializing fresh per-session state.
+        # See hermes-lcm#68. Built-in ContextCompressor ignores kwargs.
+        try:
+            _old_sid = locals().get("old_session_id")
+            if _old_sid and hasattr(self.context_compressor, "on_session_start"):
+                self.context_compressor.on_session_start(
+                    self.session_id or "",
+                    boundary_reason="compression",
+                    old_session_id=_old_sid,
+                )
+        except Exception as _ce_err:
+            logger.debug("context engine on_session_start (compression): %s", _ce_err)
+
         # Warn on repeated compressions (quality degrades with each pass)
         _cc = self.context_compressor.compression_count
         if _cc >= 2: