feat(calendar-to-brain): add ClawVisor-backed collector and privacy tests

[alexandreroumieu-codeapprentice]

Summary

Adds a first Calendar-to-Brain collector backed by ClawVisor task output.

This introduces a safe local workflow for turning Google Calendar events into dated Brain source pages under:

~/brain/sources/google-calendar/YYYY/YYYY-MM-DD.md

What changed

• Adds integrations/calendar-to-brain/collector.mjs.
• Adds a mock ClawVisor response fixture for local tests.
• Groups calendar events by day.
• Skips cancelled events.
• Renders daily markdown pages with YAML frontmatter.
• Redacts the Google Calendar service identifier in generated markdown.
• Avoids leaking attendee email addresses into generated markdown.
• Keeps the CLI safe by default: no write unless explicitly requested.

Validation

bun test test/calendar-to-brain.test.js

Expected result:

5 pass
0 fail

Notes

This PR supersedes the older draft #751 with a smaller first slice.

This PR only introduces the base collector and privacy/safety tests. Preview UX, overwrite protection, and French human-readable dates are intentionally split into follow-up PRs.

[alexandreroumieu-codeapprentice]

check:wasm appears to fail on Bun’s compiled-binary embedded-WASM runtime path (also reproducible on master). PR #790 only touches calendar-to-brain files and does not modify that codepath.