Add spawn_agent: sub-agent delegation with isolated context ...

- Agent.run_ephemeral() — runs a sub-agent loop without a persistent
  session; accepts exclude_tools to block recursion; logs start/complete
- session_store made Optional in Agent.__init__ (None for ephemeral runs)
- SpawnAgentTool (navi/tools/spawn_agent.py): spawns an isolated Agent
  for a focused task; resolves profile from parent session via ContextVar;
  blocks spawn_agent recursion via exclude_tools=["spawn_agent"]
- build_default_registries() accepts session_store param; registers
  SpawnAgentTool after BackendRegistry is built (patches _backend_registry)
- deps.py passes _session_store to build_default_registries
- All profiles: spawn_agent added to enabled_tools, max_iterations 10→30
- persona.txt: DELEGATION section — when/how to use spawn_agent

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>