feat(reply): render Markdown bodies as HTML

[kyoji2]

Summary

• add explicit --body-format markdown to conversation and ticket replies
• render both inline and file bodies through shared markdown-it configuration with source HTML disabled and soft line breaks preserved
• cover raw compatibility, invalid formats, help, dry-run payloads, message-type precedence, and Markdown input forms

Verification

• bun test (62 pass, 0 fail)
• bun run lint
• bun run typecheck
• bun run build
• git diff --check

Notes

• AGENTS.md update was attempted but blocked by the protected-instruction-file approval policy; README documents the runtime convention instead.