spawn_agent: fix model behavior — synthesis reminder + lower delegation threshold ...

- ToolResult.output now prefixed with explicit reminder that the user
  cannot see sub-agent output and the orchestrator must present findings
  in its own final response; reminder appears adjacent to the result so
  the model reads it in context
- spawn_agent description updated: lowers threshold from "3+" to "2+"
  tool calls, makes USER CANNOT SEE warning prominent, tightens wording
- persona.txt DELEGATION section: clearer rule (default to spawning for
  multi-step sub-tasks), explicit "never end turn after spawn results",
  removed abstract threshold in favour of concrete examples

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

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>

Add long-term user memory system ...

Architecture:
- navi/memory/store.py: MemoryStore backed by SQLite (memory_facts,
  memory_summary, session_memory_state tables in navi.db)
- navi/memory/extractor.py: LLM-based fact extraction from sessions +
  summary regeneration (triggered after session goes idle >30 min)
- Fact upsert uses UNIQUE(category, key) — same key always overwrites,
  no duplicates or stale contradictions
- Keyword search across category + key + value (LIKE-based, no extra deps)

Context injection:
- Memory summary injected as an ephemeral system message on every LLM call
  via Agent._with_memory() — never persisted to session.context

Tools (all profiles):
- memory_search(query): keyword search against fact DB; persona instructs
  model to call it at session start and before personal-context questions
- memory_forget(key, category?): delete a specific fact on user request

Extraction trigger:
- On new session creation, fire-and-forget background task checks all
  sessions idle >30 min with unprocessed messages → runs extraction

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

Fix NAVI_PERSONA not loading: move persona to persona.txt ...

python-dotenv cannot parse multi-line env values that contain blank lines or
lines resembling key=value assignments (like the write_tool code examples).
The persona was silently dropped, leaving Navi with no identity.

Solution: navi_persona_file setting (default empty) — if set, the persona is
read from the specified file at startup via a model_validator. The .env now
points to persona.txt instead of embedding the multi-line string.

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