feat: add tag-scoped statistics

[vycdev2]

Summary

• add tag-scoped statistics tokens for counts, completion, estimates, and elapsed time
• support AND/OR selectors such as [tag:frontend&high:pending] and [tag:frontend|backend:done]
• document the syntax and cover global/project aggregation, rendering, and lazy collection

Verification

• npm run check — passed: type checking, 405 tests, development compilation, and Prettier checks
• git diff --check — passed

Risk

• Medium: extends statistics aggregation and template rendering; collection is disabled unless a tag-scoped token is configured, and existing tokens remain unchanged.

Closes #81

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