cleanup: remove dead artifacts (#21)

refactor(cache): centralize health cache path consumers

chore(src): remove deprecated claude_headless_stream client

chore(src): remove unused ai_validation module

refactor(garmin-sync): remove unused Garmin fetch helpers

docs(skills): clarify garmin sync pipeline role

refactor(discord): extract shared pending-state sqlite helpers

refactor(llm): migrate off gemini_client module

feat(brain): add public call_gemini compatibility shim

chore(vault): remove unused read_daily_note

chore(garmin-sync): remove dead supplemental import

fix(bot): use DST-aware timezone for all scheduled tasks

fix(macro): use last 7-day non-race mileage instead of 14-day rolling avg

remove: delete strength, nutrition, and mobility coaching features

docs(claude): add refactoring rules for import changes and test verification