feat(scanners): schedule weekly workspace friction scan

[solomonneas]

Summary

• Add weekly@HH:MM scanner cadence (due after 7 days) and cadence-aware stale thresholds (192h for weekly) so weekly scanners do not WARN mid-cycle.
• Enable a default weekly workspace friction-scan (brigade friction scan --json, weekly@05:00, output .brigade/friction/latest.json) with a conflict window that clears security/backup slots.
• Document the cadence in docs/scanner-registry.md and cover parse/due/defaults/stale thresholds in tests.

Fixes #481

Test plan

• brigade work verify run --target . --command "./scripts/verify" (exit 0)
• Focused scanner tests for weekly parse, due, defaults, and stale hours

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