feat(ai): support GitHub Copilot CLI

[ChrisLauinger77]

What & why

Add GitHub Copilot CLI as an installed AI provider. AngKorGit now detects the copilot binary—including WinGet installations on Windows—and invokes it in silent, non-interactive mode using the user’s existing Copilot login and quota. Optional model overrides are supported, and the settings UI, documentation, website, allowlist, and tests have been updated accordingly.

Screenshots

Not included—this only adds provider text/list entries and does not change styling or colors.

Checklist

• pnpm typecheck && pnpm test pass
• cargo fmt --check && cargo clippy --all-targets -- -D warnings && cargo test pass (Rust toolchain unavailable locally)
• New engine functions have integration tests in tests/git_engine.rs — not applicable; no new engine function
• Mutating operations go through the undo tracked() wrapper where undoable — not applicable
• Colors use design tokens only (no styling changes)
• CLAUDE.md updated
• Commits carry no coding-tool trailers