cmd/git(feat[GitTag]): add GitTagManager and GitTagCmd classes

why: Enable programmatic tag management following Manager/Cmd pattern
what:
- Add GitTagCmd for per-tag operations: delete, verify, show
- Add GitTagManager for collection-level operations: create, ls, get, filter
- Expose git.tags on Git class
- CLI flag mappings: -a (annotate), -s (sign), -f (force), -m (message)
- Support ls filters: pattern, sort, contains, merged

Author: tony