Summary

CLI half of fangorn/anvil#315. anvil pr edit could only change title/body; this adds the remaining editable PR metadata.

Changes

• --base <branch> — repoint the base branch (sends base_branch).
• --draft / --ready — toggle is_draft (mutually exclusive via clap conflicts_with).
• Updated the empty-payload hint and the subcommand doc string.
• On a base change the success output echoes the new base.

Requires the server change in fangorn/anvil#179 (PR update endpoint now accepts base_branch/is_draft).

Tests

Added parse tests: title+body, base repoint, --draft, --ready, and that --draft --ready together is rejected. cargo test commands::pr: 9 passed. cargo fmt --check and cargo clippy --all-targets -- -D warnings clean.

Refs fangorn/anvil#315