[Prototype] Add signed local mactools CLI and capability API

[xcv58]

Prototype status

This draft PR is a working prototype for RFC #309. It is intended to make the packaging, security, IPC, lifecycle, and command-surface decisions testable; it does not propose freezing every implemented command into CLI v1.

What this prototype includes

• A separately downloadable, signed universal mactools-cli release archive whose installed command is mactools; the CLI executable is not embedded in MacTools.app.
• An opt-in, user-scoped LaunchAgent/XPC broker provided and registered by the app.
• A narrow broker responsible for peer authentication, protocol negotiation, host startup, forwarding, admission bounds, and cancellation.
• Host-owned action discovery and execution through the existing Action Registry and Action Executor.
• Exact role identity, Team ID, Apple anchor, and effective-user trust checks, with separate development service identities.
• Versioned request/response contracts, bounded cold start, structured JSON output, stable exit codes, cancellation, and upgrade/failure recovery.
• A broad prototype command surface covering action discovery/execution, typed and sensitive input transport, workflow conveniences, and read-only plugin diagnostics.
• Separate release packaging, notarization, checksums, artifact validation, and Homebrew-ready distribution support.

Decisions reflected from the RFC discussion

• The CLI is optional and distributed separately while sharing the app release cadence.
• The app provides the broker; the CLI package contains only the command-line executable.
• The broker does not load plugins or execute actions.
• MacTools remains the authoritative source for installed plugins, availability, permissions, confirmation, concurrency, timeouts, and execution.
• Run Links remain supported and no network or public remote-control API is introduced.

Areas to validate before defining v1

• Which of the broader prototype commands should remain in the first public release.
• Whether CLI eligibility should be fully independent from Run Link externalInvocationPolicy or retain it as an additional conservative gate.
• Installation, upgrade-ordering, background-item approval, and removal ergonomics.
• The appropriate initial policy for safe, interactive, confirmation-required, and parameterized actions.

Verification

The branch includes unit, executable, protocol, packaging, release-layout, and end-to-end coverage. The latest response-contract changes were verified with:

xcodebuild -project MacTools.xcodeproj -scheme MacTools -configuration Debug \
  -derivedDataPath build/DerivedData test -quiet \
  -only-testing:MacToolsTests/CLIParameterInputTests \
  -only-testing:MacToolsTests/CLIExecutableTests

Related to #309.