feat: support GitHub Enterprise pull requests

[cosentinode]

Summary

• derive and validate GitHub host and repository identity from HTTPS and SSH remotes
• scope gh authentication, repository lookup, API queries, existing-PR discovery, and PR creation to the verified host
• preserve GitHub.com behavior while failing closed with host-specific guidance

Verification

• cargo fmt --all --check
• cargo clippy --locked --workspace --all-targets -- -D warnings
• cargo test --locked --workspace

Fixes #50

[cosentinode]

Fresh review of current head b4692cd: zero concerns. I independently inspected issue #50, the complete PR diff and surrounding GitHub CLI and pull-request planning/execution code, current checks, and tests. I also verified the real gh repository argument contract and ran cargo fmt --all --check, cargo clippy --locked --workspace --all-targets -- -D warnings, and cargo test --locked --workspace successfully. Current Rust and GitGuardian checks are green.