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>