[captcrock]

Stacked on #109 (TD-5223). This PR targets td/5223-stop-file. Once #109 merges, retarget it to main.

What

A system-wide shortcut that stops computer use on the whole machine, even while TST Desk is in the background. ⌘. / Ctrl+. only works when TST Desk is the front window, and during computer use the agent is usually in another app.

• Default Ctrl+Alt+Super+Period (⌃⌥⌘. on macOS), set by computer_use.stop_shortcut; empty turns it off. Read at launch. It doesn't take ⌘. system-wide, since many apps use that for Cancel; the in-window ⌘. is unchanged.
• Same stop-file as the title-bar Stop (TD-5223), so every computer-use client halts, including ones TST Desk didn't start. The app writes the file itself, so the shortcut works while the daemon is busy; the daemon's once-a-second check then updates every window.
• Engage only. It never removes the file. An agent can press key combos, so a toggle would let it switch itself back on. Resume stays a click in TST Desk.
• One global-shortcut plugin. It's now built once in lib.rs, and each shortcut registers its own handler (on_shortcut). Quick entry (macOS) moved to that too; before, it built the plugin with a single handler.
• Config model, shipped config.yaml, Settings help text, docs/configuration.md (plus a short section on the machine-wide Stop), DECISIONS (2026-11-02, TD-5224) and backlog (TD-5224).

Tests (each shown to fail when its piece is broken)

• shell stop_hotkey (6): unset → default; a custom combo; empty → off; the default parses to Ctrl+Alt+Super+. and is not ⌘.; pressing creates the stop-file at the resolved path (default, and killswitch.stop_file from the tst-cu-mcp config); pressing again while stopped keeps it stopped. Mutations checked: turning engage into a toggle, defaulting to Super+Period, and ignoring "empty is off" each fail a test.
• core test_stop_shortcut_config.py (4): model default, shipped config, empty/custom values, docs row.

Suites (Linux)

• core: 8400 passed, 12 skipped; ruff and ruff format clean; mypy --strict tstd clean
• ui: 190 files, 2722 passed; svelte-check 0 errors, 0 warnings
• shell: cargo test --lib 131 passed; cargo clippy --all-targets -D warnings clean
• mcp/tst-cu-mcp: 616 passed, 3 skipped, 3 failed: pre-existing (DarwinBackend.hit_test nested in _mark_prompted, other in-flight work)

Not verified here

• The shortcut firing on a real desktop. The installed app on the test machine is a different build, and the app is single-instance, so I didn't launch this branch next to it. The registration and press handler are covered by unit tests only.
• macOS compile of quick_entry.rs. That code is macOS-only and CI builds on Linux. It now calls the same on_shortcut API the stop shortcut uses (which builds and runs here), but the Mac build is the real check.
• Live macOS check, for the Mac session: press ⌃⌥⌘. with another app in front → the stop-file exists, an outside client's clicks are refused, TST Desk shows "CU stopped" within a second; press it again → still stopped; Resume in TST Desk clears it; quick entry (⇧⌘.) still opens.

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