feat(calendar): add ClawVisor Calendar-to-Brain collector

[alexandreroumieu-codeapprentice]

Summary

• Add a local Calendar-to-Brain collector backed by ClawVisor/credential-gateway.
• Write Google Calendar events as source markdown pages under ~/brain/sources/google-calendar/YYYY/ with calendar service redaction.
• Add fixture-based tests for dry-run/write/redaction behavior.
• Silence shipped embedding max_batch_tokens startup warnings by adding a real Google cap and explicit dynamic-limit sentinels for Ollama/LiteLLM.

Validation

• bun test test/calendar-to-brain.test.js
  • 3 pass / 0 fail
• bun test test/ai/adaptive-embed-batch.test.ts
  • 24 pass / 0 fail / 49 expect() calls
• Manual smoke: limited 7-day Calendar write + targeted GBrain import/search succeeded locally, with google.calendar:[redacted] and no email/service ID leakage.
• Manual smoke: gbrain get sources/google-calendar/2026/2026-05-11 no longer emits the embedding max_batch_tokens warnings.

Notes

• No Gmail sync, no cron, and no backfill are enabled by this PR.

[alexandreroumieu-codeapprentice]

Superseded by the smaller split draft PR #790. Follow-up Calendar-to-Brain safety/polish branches are prepared separately.