Add native terminal mode for Copilot sessions

[Copilot]

• Inspect existing session keybindings and prior commits
• Run baseline validation for relevant Rust checks
• Add or document a shortcut for leaving native sessions without quitting
• Run targeted/full validation after changes
• Reply to PR comment with the shortcut or commit hash

[joshblack]

@copilot can you keep a shortcut around for leaving the session? (without quitting)

If it already exists, leave a comment telling me what it is

[Copilot]

@copilot can you keep a shortcut around for leaving the session? (without quitting)

If it already exists, leave a comment telling me what it is

For native terminal mode, use tmux detach: Ctrl-b, then d. That leaves the Copilot session running and returns to gh-pilot. In embedded preview mode, the detach shortcut is Ctrl-w.