Add AIHelper + filesystem query/smart_edit AI actions ...

AIHelper (navi/core/ai_helper.py):
- Reusable LLM utility for AI-enhanced tools: ask() and ask_json()
- Reads current_model ContextVar (set per-turn) so tools always use
  the session's active model without extra wiring
- Robust JSON extraction: strips markdown fences, bracket-matching fallback

current_model ContextVar (navi/tools/base.py):
- New ContextVar set by run_stream() and run_ephemeral() after profile
  is resolved; AIHelper reads it to pick the right model automatically

filesystem query action:
- Natural language question about any file, chunked at ~20k tokens of
  content (~80k chars) with 30-line overlap between chunks
- Single-chunk: one LLM call; multi-chunk: partial answers accumulated
  then synthesized in a final call

filesystem smart_edit action:
- Natural language edit instruction on files up to ~200k chars
- LLM outputs JSON patch ops: replace / delete / insert (1-based lines)
- Ops validated then applied bottom-up to preserve line numbers
- Returns unified diff of changes; preserves trailing newline

registry: AIHelper created once, OllamaBackend reused (no double init),
FilesystemTool receives ai_helper at construction

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

Improve filesystem, web search, context guard, and subagent narration ...

filesystem: add find (glob), info (stat), move, append actions; read now
supports offset/limit with hard 1MB guard; list shows sizes, dates,
optional recursion.

web_search: retry DDG across auto/html/lite backends; add optional Brave
Search API and SearXNG fallbacks configured via .env.

agent: fix ContextTooLargeError to surface as Navi response instead of
raw system error; fix _check_context_size to calculate from remaining
budget (window - output_reserve) rather than a fixed 92% threshold.

persona: add ReAct narration instruction to subagent briefing template.

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

Wire self-extension tools into all profiles; improve tool descriptions ...

All profiles now include write_tool, list_tools, tool_manual, reload_tools.
User tools from enabled.json merged in at runtime via Agent._tool_list().
Built-in tool descriptions rewritten to be more LLM-actionable.

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

Filesystem tool: unrestricted mode by default ...

FS_ALLOWED_PATHS=* (default) allows any path, consistent with terminal.
Path check is now lazy (per-request) instead of module-level,
so config is always fresh after restart.
~ expansion added.

Initial implementation of the agent system core ...

- FastAPI server with REST API and WebSocket streaming
- Modular LLM backend abstraction (Ollama implemented, OpenAI stub)
- Tool system: web_search (ddgs), filesystem, http_request, code_exec, terminal
- Agent profiles: smart_home, server_admin, secretary
- Tool-calling loop with concurrent tool execution
- In-memory session store with SessionStore ABC for future persistence
- Registry pattern for tools, profiles, and backends
- Orchestrator stub as foundation for multi-agent scenarios