Simplify skill curation and suggestion defaults

[aktasbatuhan]

Summary

• make full skill curation manual by default while preserving explicit scheduling
• isolate the entire pytest session from live Watchmen state
• require stronger positive trigger overlap and honor negative triggers
• index only skills that are actually installed, refreshing on install changes

Validation

• 575 tests passed
• Python syntax compilation passed
• git diff --check passed
• live ~/.watchmen/state.db checksum was unchanged across test runs

Follow-up

Hook lifecycle simplification will be handled separately so this safety patch has an independent rollback boundary.