feat: Add @mention formatting for comments and checklists #20

[steveclarke]

Summary

• Adds MentionFormatter class that converts {{@Username}} syntax to Superthread's <user-mention> HTML tags
• Integrates at the resource layer so both CLI and library consumers get automatic mention support
• Applies to comments (create/update), replies (create/update), and checklist items (add/update)

Ports the existing implementation from the TypeScript MCP server (mcp-superthread-plus/src/utils.ts).

Closes #20

Details

• Three-pass algorithm: escape protected mentions, replace matches, restore escaped text
• Case-insensitive name matching against workspace members
• HTML-escapes attribute values for XSS defense
• Graceful degradation: returns content unchanged if member fetch fails
• Unmatched names stay as literal {{@Name}} text

Test plan

• 15 unit tests covering all behaviors (match, no match, escaping, XSS, error handling)
• Full suite passes (724 examples, 0 failures)
• StandardRB clean
• YARD docs clean
• Manual verification: created a comment with {{@Steve Clarke}} on MCP Testing board, confirmed <user-mention> tag in API response