Implement agenting skills support via Vercel Labs' skills

[albertodebortoli]

Add skills support to Luca

Extends luca install to install agentic SKILL.md-based skills from Git repositories, alongside existing binary tool management.

Description

• New skills: key in Lucafile: Declare agentic skills hosted in Git repositories. Each skill entry takes a repository (owner/repo shorthand or full HTTPS/GIT URL) and an optional name. Omitting name installs all skills from that repository.
• New agents: key in Lucafile: Optional list of agent identifiers (e.g. claude-code, github-copilot, opencode) of supported agents to target when installing skills. When omitted, skills are installed for all supported agents.
• --only-tools / --only-skills flags: Selective install modes on luca install to run only one half of the pipeline. Mutually exclusive; validated at argument parse time.
• SkillInstaller: New component behind a SkillInstalling protocol. Builds and runs npx skills add <repository> --yes [--skill name...] [--agent name...] via SubprocessRunner. Verifies npx availability before proceeding and maps non-zero exit codes to typed errors. skills is a tool developed and maintained by Vercel Labs.
• SubprocessRunner: New SubprocessRunning abstraction wrapping Foundation.Process. Uses terminationHandler + withCheckedThrowingContinuation (instead of waitUntilExit()) to avoid deadlocking on Swift Concurrency threads. Closes stdin via FileHandle.nullDevice so subprocesses do not block waiting for input.
• SkillsInfoFactory: Groups Skill entries from the spec by repository into SkillSet batches, merging per-repo entries (a repo listed without a name takes precedence and installs all its skills).
• ToolInstaller (refactor): Extracts the per-tool download, validate, and install pipeline out of Installer into a dedicated ToolInstaller struct behind ToolInstalling. This creates a symmetric architecture — one installer per domain — and leaves Installer as a pure orchestrator (spec loading, orphan unlinking, delegation).
• reinstall moved to ToolInstaller: All tool-specific dependencies (binaryFinder, permissionManager, symLinker) already lived in ToolInstaller; reinstall moved there too, shedding those stored properties from Installer.

Lucafile example:

tools:
  - name: SwiftLint
    version: 0.61.0
    url: https://github.com/realm/SwiftLint/releases/download/0.61.0/portable_swiftlint.zip

skills:
  - name: frontend-design
    repository: vercel-labs/agent-skills
  - name: swift-testing-expert
    repository: https://github.com/AvdLee/Swift-Testing-Agent-Skill

agents:
  - claude-code
  - github-copilot

Type of Change

• Feature
• Bug fix
• Maintenance / Refactor
• Documentation
• CI / Tooling
• Other (specify)

How Has This Been Tested?

• Added / updated unit tests
• Manually tested locally (describe)
• Tested on macOS (arch: arm64 / x86_64)
• Other

New test files:

• Tests/Core/SubprocessRunnerTests.swift — verifies SubprocessRunner against real /bin/true and /bin/false executables
• Tests/Core/SkillInstallerTests.swift — covers npx availability check, argument construction, and error paths via SubprocessRunnerMock
• Tests/Core/SkillsInfoFactoryTests.swift — covers spec-based grouping, "all skills" vs named skills merging, and empty/nil cases
• Tests/Core/ToolInstallerTests.swift — full pipeline tests (zip/tar.gz/executable, arch validation, reinstall) extracted from the former InstallerTests, now calling ToolInstaller directly
• InstallerTests updated to use ToolInstallerMock for orchestration tests and a real ToolInstaller + mock downloader for integration tests

Screenshots / Demo (if applicable)

$ luca install
# installs both tools and skills

$ luca install --only-tools
# installs binary tools only, skips skills

$ luca install --only-skills
# installs skills only, skips tools

Checklist

• Swift code builds locally (swift build)
• Tests pass locally (swift test)
• Code style / formatting respected
• Documentation updated (README / comments)
• Version / tag alignment considered (if release related)
• PR title follows conventional style (optional)

CI Considerations

• Affects build time notably
• Requires new secrets / env vars
• Alters release process

Breaking Changes?

• No
• Yes (describe impact and migration path)

Additional Notes

npx (and Node.js) must be present in the environment for skill installation to work. If npx is not found, luca install fails with a clear error message pointing the user to install Node.js. Skills installation is silently skipped when the Lucafile contains no skills: key.

The SubprocessRunner deadlock fix (terminationHandler instead of waitUntilExit()) is required because Swift Concurrency tasks do not run a RunLoop, which waitUntilExit() depends on internally.