feat: expose HERMES_SESSION_ID to agent tools via ContextVar + env (#23847)

Set HERMES_SESSION_ID using the existing session_context.py ContextVar
system for concurrency safety (multiple gateway sessions in one process
won't cross-talk). Also writes os.environ as fallback for CLI mode.

Touchpoints:
- gateway/session_context.py: Add _SESSION_ID ContextVar + _VAR_MAP entry
- run_agent.py: Set both ContextVar and os.environ at init and on
  context-compression rotation
- tools/environments/local.py: Bridge ContextVars into subprocess env
  in _make_run_env() (ContextVars don't propagate to child processes)
- tests/run_agent/test_session_id_env.py: 3 tests covering env, provided
  ID, and ContextVar paths

execute_code subprocess already passes HERMES_* prefixed vars through
_scrub_child_env (line 82: _SAFE_ENV_PREFIXES includes 'HERMES_').

Primary use case: webhook-triggered agents that need to include a
`--resume <session_id>` takeover command in their output.

run_agent.py

@@ -1837,7 +1837,20 @@ class AIAgent:
             timestamp_str = self.session_start.strftime("%Y%m%d_%H%M%S")
             short_uuid = uuid.uuid4().hex[:6]
             self.session_id = f"{timestamp_str}_{short_uuid}"
-        
+
+        # Expose session ID to tools (terminal, execute_code) so agents can
+        # reference their own session for --resume commands, cross-session
+        # coordination, and logging.  Uses the ContextVar system from
+        # session_context.py for concurrency safety (gateway runs multiple
+        # sessions in one process).  Also writes os.environ as fallback for
+        # CLI mode where ContextVars aren't used.
+        os.environ["HERMES_SESSION_ID"] = self.session_id
+        try:
+            from gateway.session_context import _SESSION_ID
+            _SESSION_ID.set(self.session_id)
+        except Exception:
+            pass  # CLI/test mode — ContextVar not needed
+
         # Session logs go into ~/.hermes/sessions/ alongside gateway sessions
         hermes_home = get_hermes_home()
         self.logs_dir = hermes_home / "sessions"
@@ -10233,6 +10246,12 @@ class AIAgent:
                 self._session_db.end_session(self.session_id, "compression")
                 old_session_id = self.session_id
                 self.session_id = f"{datetime.now().strftime('%Y%m%d_%H%M%S')}_{uuid.uuid4().hex[:6]}"
+                os.environ["HERMES_SESSION_ID"] = self.session_id
+                try:
+                    from gateway.session_context import _SESSION_ID
+                    _SESSION_ID.set(self.session_id)
+                except Exception:
+                    pass
                 # Update session_log_file to point to the new session's JSON file
                 self.session_log_file = self.logs_dir / f"session_{self.session_id}.json"
                 self._session_db_created = False