refactor(tag-util): harden mention parsing and exclude non-actionable markdown contexts

[heoh]

resolves #3

Summary

This PR improves VC parsing reliability and removes noisy mention extraction.

Changes

• Added masking for non-actionable Markdown regions:
  • fenced code blocks
  • inline code
  • blockquote lines
• Applied masking to both title and body.
• Added safe handling for malformed/incomplete Markdown (no throw fallback).
• Refactored mention parsing to use normal-text context only:
  • header mentions (###### authored by ...) are excluded from mentions
  • command-line mentions (/assign ...) are excluded from mentions
• Deduplicated mentions while preserving first-seen order.
• Updated tests to assert exact ordered output directly (without sort helpers).