Adds `ocas tag <target> <tag>...` and `ocas untag <target> <tag>...`
top-level CLI commands operating on store.tag.* (TagStore). Targets
may be hashes or @scope/name variables (resolved via resolveHash).
The redundant `ocas var tag` subcommand is removed; `var tag` now
falls through to "Unknown var subcommand: tag".
Registers `@ocas/output/tag` and `@ocas/output/untag` schemas and
templates in bootstrap; removes `@ocas/output/var-tag`.
Closes#52
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>