Add Gh command family

[maniramezan]

Summary

• add a typed Gh command family for GitHub CLI automation
• cover PRs, repos, workflows, API calls, Copilot, skills, SSH keys, extensions, and agent tasks
• wire the Gh trait into docs, CI matrices, README, AGENTS, and the shared SwiftyShell skill

Validation

• swift build -c release --traits Gh -Xswiftc -warnings-as-errors
• swift test --traits Gh -Xswiftc -warnings-as-errors
• swift test -Xswiftc -warnings-as-errors
• swift build -c release -Xswiftc -warnings-as-errors
• swift build -c release --enable-all-traits -Xswiftc -warnings-as-errors
• swift test --enable-all-traits -Xswiftc -warnings-as-errors
• swift test --enable-all-traits --enable-code-coverage -Xswiftc -warnings-as-errors
• swift -warnings-as-errors Scripts/validate-code-coverage.swift --input .build/arm64-apple-macosx/debug/codecov/SwiftyShell.json --minimum-line-coverage 84
• swift -warnings-as-errors Scripts/validate-traits.swift
• swift -warnings-as-errors Scripts/validate-docc-coverage.swift
• swift package -Xswiftc -warnings-as-errors --allow-writing-to-directory docs generate-documentation --target SwiftyShell --output-path docs --transform-for-static-hosting --hosting-base-path SwiftyShell
• swift-format lint --strict Package.swift Sources/SwiftyShell/Gh/Gh.swift Tests/SwiftyShellTests/Gh/GhTests.swift
• git diff --check