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

tools/delegate_tool.py

@@ -251,6 +251,7 @@ def _build_child_agent(
         clarify_callback=None,
         thinking_callback=child_thinking_cb,
         session_db=getattr(parent_agent, '_session_db', None),
+        parent_session_id=getattr(parent_agent, 'session_id', None),
         providers_allowed=parent_agent.providers_allowed,
         providers_ignored=parent_agent.providers_ignored,
         providers_order=parent_agent.providers_order,