feat(cli): add changes implementation start command

Context
===
Implementation tracking in specd previously relied on implicit detection
tied to lifecycle transition into `implementing`. There was no explicit
CLI command to activate tracking on demand or persisted tracking timestamp
in `manifest.json`.

Changes
===
- add `specd changes implementation start <name>` CLI subcommand
- add `implementationTrackingStartedAt` property and
  `isImplementationTrackingActive` getter to `Change`
- add `Change.startImplementationTracking()` domain mutation method
- add `'start'` action to `UpdateImplementationTracking` use case
- auto-activate implementation tracking on `TransitionChange` to
  `implementing`
- guard detection in `RefreshImplementationTracking` and resolve
  baseline ref via `VcsImplementationDetector`
- update `manifest.json` schema and `FsChangeRepository` serialization
- add unit and integration tests across `@specd/core` and `@specd/cli`
- update `docs/cli/cli-reference.md`