Description

Convert current buffer's markdown to RTF and put it on system clipboard, so pasting into Mail/Word/Docs/Pages shows formatted text instead of raw markdown. Reuse the existing HTML export pipeline (internal/export) as the conversion source: render markdown->HTML via Document(), then convert HTML->RTF. On macOS, shell out to 'textutil -convert rtf -stdout' for HTML->RTF, then 'osascript' to push the RTF bytes onto NSPasteboard as public.rtf (plain atotto/clipboard.WriteAll only sets string type, not RTF). Non-macOS: best-effort or explicit unsupported error, matching the existing GOOS-switch pattern in internal/export/file.go browserCommand(). Trigger via a new in-app keybind (avoid existing Ctrl+D/E/F/G/I/K/L/N/P/Q/S/U/slash) and a new CLI flag mirroring -e/--export (TASK-030), both funneling through a.exportOptions() like Ctrl+E does today.

Acceptance Criteria

• #1 New function in internal/export converts already-rendered HTML (or markdown) to RTF bytes on darwin
• #2 Clipboard receives RTF-typed data on macOS: pasting into TextEdit/Mail/Word shows bold/headings/lists formatted, not raw markdown text
• #3 New in-app keybind copies current buffer to clipboard as rich text, mirroring Ctrl+E's export flow
• #4 New CLI flag (headless) performs the same conversion and writes/copies without opening the editor
• #5 Non-macOS platforms fail with a clear 'unsupported' error rather than silently writing plain text
• #6 Tests cover the HTML->RTF conversion function and CLI flag wiring (skip/guard the actual clipboard write in tests)