feat: stable task ID — position-based mapping for rewritten task detection (#65) (#73)

- Add `stableTaskID`, `loadTaskIDs`, `saveTaskIDs` in `cache.go`
- Add `buildBaseToCurrentMapping` in `reconcile.go` for position-based
text change detection
- Remove unused hash-based stable ID cache (replaced by simpler
position-based mapping)
- 3 new tests: `TestStableTaskIDWordingChange`,
`TestStableTaskIDWordingChangeUncheck`, `TestBuildBaseToCurrentMapping`
- 285 tests pass, `go vet` clean, `gofmt` clean

Author: androidand