Add Linux AppShots support

[moxw]

Summary

• enable upstream AppShots availability, capture, hotkey, and settings paths on Linux
• add a Linux AppShot backend path that captures the focused window screenshot plus AT-SPI context through bundled Computer Use
• add screenshot CLI fallbacks for grim, gnome-screenshot, spectacle, and ImageMagick import
• document Linux AppShots usage and backend commands

User-visible behavior

• Linux builds expose AppShots and attach the focused Linux window with screenshot and accessibility context
• the default Linux AppShots hotkey is both Shift keys, with both Alt keys and Ctrl+Alt+A available in settings

Validation

• node --check scripts/patches/appshots.js
• node --test scripts/patch-linux-window-ui.test.js
• cargo fmt --check
• cargo check -p codex-computer-use-linux
• cargo test -p codex-computer-use-linux
• git diff --check
• rebuilt a side-by-side app at /tmp/codex-appshots-lab-app and verified the generated app.asar includes the AppShots preview payload

[ilysenko]

Thanks for working on this. The idea is useful, but I do not think AppShots should land as an always-on core patch right now.

Please move the Linux AppShots UI exposure/settings/hotkey patches into an opt-in linux-features/appshots feature, disabled by default. This fits the repo policy better: AppShots is a useful Linux-specific integration, but not mandatory compatibility for every Linux build.

Main reasons:

• The availability patch currently bypasses the upstream gate by changing macOS && flag into linux || macOS && flag. That should not be core/default behavior.
• The screenshot path can fall back to CLI tools after portal denial/cancel, which may bypass the user/compositor consent boundary.
• AppShot can degrade from “focused window capture” into a full-screen screenshot if focused-window/crop fails.
• The default DoubleShift hotkey depends on X11/XInput tooling (xmodmap, xinput) and is not reliable enough as a default Linux-wide behavior.
• The new AppShots descriptors should not be required-upstream if this is optional; otherwise upstream chunk drift can break fresh builds for a non-essential feature.

I would be happy with this as an optional linux-features/appshots integration after the privacy/fail-closed cases are tightened.

[moxw]

Thanks, addressed in the latest push.

What changed:

• Moved the AppShots main-process, availability, settings, and hotkey patch descriptors out of core and into disabled-by-default linux-features/appshots
• Left the backend appshot command available, but made AppShot screenshot capture fail closed when focused-window bounds are missing, bounds do not intersect the screenshot, or GNOME Shell / XDG Portal capture fails
• AppShot capture no longer uses the CLI screenshot fallback path
• Removed the Linux default global hotkey; opt-in users can configure a hotkey from settings after enabling the feature
• Updated README / CHANGELOG and added feature-level tests

Local validation passed:

• node --test scripts/patch-linux-window-ui.test.js
• node --test linux-features/*/test.js
• cargo fmt --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo check --workspace --all-targets
• cargo test --workspace --all-targets
• tests/scripts_smoke.sh
• git diff --check