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

run_agent.py

@@ -530,6 +530,7 @@ class AIAgent:
         skip_context_files: bool = False,
         skip_memory: bool = False,
         session_db=None,
+        parent_session_id: str = None,
         iteration_budget: "IterationBudget" = None,
         fallback_model: Dict[str, Any] = None,
         credential_pool=None,
@@ -1025,6 +1026,7 @@ class AIAgent:
         
         # SQLite session store (optional -- provided by CLI or gateway)
         self._session_db = session_db
+        self._parent_session_id = parent_session_id
         self._last_flushed_db_idx = 0  # tracks DB-write cursor to prevent duplicate writes
         if self._session_db:
             try:
@@ -1038,6 +1040,7 @@ class AIAgent:
                         "max_tokens": max_tokens,
                     },
                     user_id=None,
+                    parent_session_id=self._parent_session_id,
                 )
             except Exception as e:
                 # Transient SQLite lock contention (e.g. CLI and gateway writing