Add anchored markdown comment utilities

[machado144]

Summary

• add a new comment command group for hidden-anchor markdown PR comments
• support rendering, fencing, inspecting, selecting, and amending anchored comment bodies
• upgrade fenced code block parsing to support longer fences with nested triple backticks
• document a gh-based PR comment upsert workflow

Tests

• go test ./...
• go build ./...
• go run . comment render --anchor preview '## Preview\n\nReady'
• printf 'before\nyaml\na: b\n\nafter\n' | go run . comment fence --language md | go run . parse extract-block --language md
• printf '[{"id":42,"html_url":"https://example.test/c","user":{"login":"bot"},"body":"\n\nready"}]' | go run . comment select --anchor preview --format id