Harden sync engine, fix completion bug, remove Habitica

[deeqdev]

Summary

• Fix completed-task data loss: Completing a task on one provider no longer deletes it from all providers. Completed status now syncs via field-level update, so the task remains visible as "completed" on both sides.
• Remove Habitica: Stripped the Habitica provider, tests, config, env vars, and docs. Only Google Tasks and Microsoft To Do remain.
• Server-side filtering: Google provider uses updatedMin, Microsoft uses $filter on lastModifiedDateTime — avoids fetching full task lists when only recent changes are needed.
• Cleaner reports: Noop actions are counted but no longer emitted in the actions array, keeping sync reports concise.
• Prior hardening (from earlier commits): delete-wins tombstone propagation, tombstone TTL pruning, lockfile, state backup, state versioning + migration, field-level conflict merge with canonical baseline, HTTP retry/backoff with 429 Retry-After, graceful degradation when a provider is down.

Test plan

• All unit tests pass (11/11)
• Typecheck clean
• Lint clean
• Build clean
• E2E tested against live Google Tasks + Microsoft To Do APIs (36 assertions)
• Manual testing: create/edit/complete/delete tasks and verify sync