* feat(tools): add global learnings store with CRUD library and CLI support
* fix(tools): address review feedback for global learnings store
- Validate learning IDs against path traversal in learningsRead, learningsDelete, and cmdLearningsDelete
- Fix total invariant in learningsCopyFromProject (total = created + skipped)
- Wrap cmdLearningsPrune in try/catch to handle invalid duration format
- Rename raw -> content in readLearningFile to avoid variable shadowing
- Add CLI integration tests for list, query, prune error, and unknown subcommand
