Make the shared client a superset of what todo's and diet_guard-app's local
GitHubClient copies provide, so every app can drop its copy and sync through
one client. canAccessRepo() is a lightweight connection test (bare repo GET,
never throws); deleteFile() resolves the file's own sha and no-ops when
absent. Mirrored on the Dart and Python sides. 90 Dart + 78 Python tests, 100%.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019KwUCPESvE5CjCtmUFz8xQ
Extracted and generalized from diet_guard's existing tombstone-merge scheme
and GitHub Contents API client, extended with per-field HLC LWW so the same
library can also cover todo's mutable notes and wake_alarm's single-value
sync (Workstream D scoping decision, 2026-07-04). 100% branch coverage,
clean ruff/mypy/pylint/bandit.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017McuA1nAi6jBT1EaS5BrVf
