Add Linux (Ubuntu) build, test, and CLI support

[devin-ai-integration]

What changed

Makes the package compile, run its full unit test suite, and pass deterministic preflight on Ubuntu (Swift 6.0.3), and adds an ubuntu-latest CI lane. The macOS interaction engine is unchanged; Linux is a protocol/tool-layer port:

• All Apple-framework code (AppKit, ApplicationServices, CoreGraphics, ScreenCaptureKit, Carbon, IOKit, Quartz) is gated with #if os(macOS) / conditional imports; macOS code paths are syntactically unchanged.
• New LinuxSupport.swift provides inert type/constant shims (AXUIElement, CGEvent, AX constants, virtual key codes) so shared code compiles; new LinuxStubs.swift reimplements the platform API surface — portable logic (keymap parsing, delivery-fallback reasoning, unicode typing chunks, page-probe JSON parsing) works for real, while anything requiring AX/input/capture throws a structured ToolError.failed("... unsupported on Linux.") rather than fabricating success (including deliverClick/deliverScroll/deliverDrag, which fail loudly on Linux).
• Daemon works on Linux: Glibc vs Darwin socket branches (sockaddr_un/AF_UNIX type differences), Linux auth-token generation, and peer authentication via getsockopt(SO_PEERCRED) (macOS keeps getpeereid); verified with a daemon round trip on Ubuntu.
• health_report --json on Linux reports platform identity with Accessibility/Screen Recording as unsupported, capture skipped, TCC not applicable; list_apps returns an empty controllable-app list.
• Linux withTimeout uses an actor-guarded continuation race so an uncancellable wedged operation cannot block timeout return (this was hanging swift test).
• ComputerUseFixture (AppKit/SwiftUI) is gated to macOS with a stub Linux main.
• CI: new linux-build-and-test job (ubuntu-latest, swift-actions/setup-swift@v2 pinned to 6.0.3, SwiftPM cache) running scripts/preflight.py; existing macOS lane untouched. README updated with the Linux support boundary.

Type of change

• New feature / capability
• Bug fix
• Docs only
• Refactor / internal (no behavior change)
• Safety-policy change

Verification

All run on Ubuntu 22.04, Swift 6.0.3 (x86_64-unknown-linux-gnu); macOS lane verified via hosted CI (green):

• swift build — Build complete
• swift test — 398 tests passed
• CLI smoke: version, help, health_report --json — all pass; health report correctly shows Linux unsupported capabilities
• python3 scripts/preflight.py --bin .build/debug/computer-use-mcp — passed (incl. Python helper tests, version consistency, background-eval dry run)
• MCP serve over stdio: initialize, tools/list (30-tool catalog), tools/call — AX/input/capture tools return structured unsupported errors, no crashes/fake success
• Linux daemon round trip: call health_report '{}' routes through the daemon; peer auth accepts same-uid clients via SO_PEERCRED
• Local live verification: not applicable — no macOS runtime behavior changed and no macOS machine available; live Linux interaction is intentionally unimplemented (tools return structured unsupported errors).

Checklist

• One logical change; commits are granular and messages explain the why.
• Tool schemas stay backwards-compatible (or the break is called out).
• Safety-policy changes include tests for both the gated and the confirmed/recovery path. (N/A — no safety-policy change; Linux stubs fail closed.)
• No default safety gate, app lease, or daemon isolation was disabled to make a test pass.
• No live GUI/TCC/input checks added to the default suite or hosted CI.
• Relevant docs updated (README / CHANGELOG / contracts under docs/).

Link to Devin session: https://app.devin.ai/sessions/5f22c5a8af704f368b963a71f94eb46f
Requested by: @minghinmatthewlam

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[devin-ai-integration]

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[devin-ai-integration]

Ran the full Linux self-test battery on Ubuntu x86_64 (Swift 6.0.3), headless shell verification per AGENTS.md non-mutating path.

Results (latest, after daemon-auth fix)

• swift build and swift test — 398/398 tests pass
• scripts/preflight.py --bin .build/debug/computer-use-mcp — all 8 steps pass ("passed": true)
• CLI version / help / health_report --json — health report correctly shows Accessibility/Screen Recording unsupported, capture skipped, "Linux does not use TCC"
• MCP serve over stdio — initialize, tools/list (full 30-tool catalog with schemas), and tools/call all respond; tools requiring AX/input/capture (type_text, open_app, page, read_clipboard, …) return structured isError:true "unsupported on Linux" errors — no crashes, no fake success
• Daemon unix-socket path — initially rejected all peers (stubbed getpeereid); fixed with SO_PEERCRED peer-credential checking and re-verified: call health_report '{}' round-trips through the daemon with no untrusted-peer rejections
• Also fixed during testing: garbled stub error rendering and placeholder rows in list_apps output

health_report --json (Linux posture)

"permissions": {
  "accessibility":   {"granted": false, "status": "unsupported"},
  "screenRecording": {"granted": false, "status": "unsupported"}
},
"captureService": {"status": "skipped", "detail": "Screen capture is unsupported on Linux."},
"tccAttribution": "Linux does not use TCC."

(Headless CLI/server project — all evidence is terminal output.)

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.