Add external prompt editor shortcut

[gachikuku]

Summary

• add a Pi-style external prompt editor action, bound to Alt+E by default
• resolve the editor from $VISUAL, then $EDITOR, with platform fallbacks
• suspend Textual while the editor owns the terminal and restore the edited prompt only after a successful exit
• expose the remappable external_editor keybinding in ~/.tau/tui.json
• preserve large-paste content, clean up temporary Markdown files, and document the workflow

Testing

• uv run pytest -q — 1295 passed, 2 skipped
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy src