Add unit tests for an untested service in Agent/Services/

[AgentiLoop]

What

AgentTests/ has 16+ test files (e.g. ScriptServiceTests.swift, DiffToolsTests.swift, ShellSafetyServiceTests.swift), but many services in Agent/Services/ still have no coverage. Pick one and add a test file.

Recipe

1. Clone and build: ./build.sh (see CONTRIBUTING.md — Xcode Command Line Tools only)
2. Browse Agent/Services/ and pick a service with no matching file in AgentTests/
3. Copy the structure/style of an existing test (e.g. ScriptServiceTests.swift)
4. Add your new file to the AgentTests target in Agent.xcodeproj
5. Run the test suite in Xcode and make sure everything passes

Definition of done

• One new *Tests.swift file in AgentTests/ covering a previously untested service, all tests green

Great first contribution if you want to learn the codebase — tests are the best map.