feat: add Claude Code skill for managing markco comments

[lesandiz]

Summary

• Adds a Claude Code skill (skills/markco/SKILL.md) that gives AI agents the ability to manage markco comments in markdown files — add, reply, resolve, edit, remove, and list comments
• Includes a zero-dependency Node.js CLI script (skills/markco/scripts/markco.mjs) that handles deterministic operations ported from CommentService.ts: anchor position calculation, code-context detection (backtick fences/inline code), --> sanitization, and block parsing/writing
• The script provides 4 subcommands: parse, serialize, find-anchor, and reconcile
• The skills/markco/ directory can be copied directly into a project's .claude/skills/ without renaming
• Updates README with setup instructions and usage examples (slash command and conversational)

How it works

The skill splits responsibilities:

• Script handles the error-prone parts: computing exact line/char positions, detecting code contexts, sanitising --> sequences, and reading/writing the markco JSON block
• AI agent handles the business logic: deciding what comments to add/edit/remove, formatting output for the user, resolving ambiguous references

Test plan

• parse correctly extracts all comments from a file with existing markco comments
• find-anchor returns correct 0-based line/char positions matching the extension's stored data
• reconcile detects orphaned anchors when text has been removed from the file
• Round-trip parse → serialize preserves the file (no unintended changes)
• parse on a file with no markco block returns {"version": 2, "comments": []}
• Code-context detection correctly skips <!-- markco-comments inside code fences
• Verify the VS Code extension still renders comments correctly after script modifications

🤖 Generated with Claude Code

[babonet]

Thank you for your PR,

1. Cross-platform command usage in SKILL.md and README
   • The skill currently instructs uuidgen, date -u, and cp -r, which are Unix-specific and will fail on Windows.
   • Recommendation: make ID/time generation platform-neutral in Node (crypto.randomUUID(), new Date().toISOString()), and avoid shell-specific copy examples.
2. Fragile JSON piping with echo '' | ... serialize
   • Single-quote based echo piping breaks when content includes apostrophes and can corrupt payloads.
   • Recommendation: use temp-file input (serialize < tmp.json) or add a --data-file option.
3. findFirstAnchor behavior diverges from extension parity
   • In skills/markco/scripts/markco.mjs, reconcile uses findFirstAnchor, which skips code-context matches if the first match is in code context.
   • In CommentService.findAnchorInDocument, the first indexOf hit is used without code-context filtering.
   • Impact: script reconciliation can mark orphaned or relocate differently from extension behavior.
   • Recommendation: either align script behavior with extension or explicitly document this as an intentional improvement.
4. Trailing-newline handling differs from extension output
   • replaceBlock trims trailing newlines before append; extension appends at end-of-file position without trimming.
   • Impact: not functionally harmful, but can produce formatting diffs and non-identical round trips.
     Parse error handling differs