`U` in the terminal's workspace asks before it stops mirroring to the source, naming the source and saying there is no way back #7986

Intent: the terminal's one-key way out of a mirror asks first; today U in src/tui/screens/workspace.ts calls stopMirroring on the keypress, and src/actions/mirror_target.ts says it is "the only way out of a mirror".

Done looks like:

• [ ] U pushes confirmScreen (the one src/tui/screens/deploys.ts uses for Stop): title "Stop mirroring", body Stop mirroring <owner>/<name> to <source>? Nothing more is pushed there, and this cannot be turned back on from here. with the source read from mirrorLine(getMirrorTarget(...)), actions Cancel (ghost) and "Stop mirroring", Cancel selected first.
• [ ] Only choosing "Stop mirroring" calls stopMirroring; Cancel or esc changes nothing; the existing toasts ("stopped mirroring" / "not mirrored") are kept.
• [ ] The key line still offers U only to an admin of a mirrored repository.
• [ ] A new test/tui-workspace-unmirror.test.ts proves: U then Cancel leaves the mirror target in place; U then confirm removes it; a non-admin is not offered U. It fails without the change.
• [ ] bun test test/tui-workspace-unmirror.test.ts test/tui-workspace.test.ts passes.

Files: src/tui/screens/workspace.ts, test/tui-workspace-unmirror.test.ts Risks: workspace.ts is large; keep the edit to the U branch. Brief: a builder who knows the terminal screens.