Add unit tests for internal/gh and internal/docker

[ewega]

Problem

internal/gh (wraps gh repo list and gh api) and internal/docker (wraps docker compose up/down/build) have zero test files. While they're thin wrappers over external CLIs, they contain output parsing logic that could break silently if the CLI output format changes.

Lower priority than internal/devlake tests (#96) but still valuable for catching regressions during refactoring.

Scope of Changes

internal/gh/gh_test.go

• Test ListRepos output parsing (newline-separated nameWithOwner)
• Test GetRepoDetails JSON parsing
• Test IsAvailable with/without gh in PATH (may need test helper)

internal/docker/docker_test.go

• Test command construction for ComposeUp, ComposeDown, Build
• Test that service list is passed correctly
• Test that --build flag is included when build=true

Note: These tests should validate command construction and output parsing, not actually invoke Docker/gh (which would require those tools to be installed).

Acceptance Criteria

• go test ./internal/gh/... passes
• go test ./internal/docker/... passes
• Tests don't require Docker or gh CLI to be installed (mock or check command construction)
• go build ./... and go vet ./... pass

[ewega]

Agent context: Create new test files internal/gh/gh_test.go and internal/docker/docker_test.go. Tests should validate command construction and output parsing — do NOT actually invoke Docker or gh CLI. Use table-driven tests (Go convention). For gh, test ListRepos output parsing (newline-separated) and GetRepoDetails JSON parsing. For docker, test command construction for ComposeUp/ComposeDown/Build and verify flags like --build are included correctly. Build with go build ./... and test with go test ./....