Add repo update command

[cursor]

Summary

Add repo update to change settings on an existing repository via PATCH /repos/{owner}/{repo}.

Motivation

Completes CRUD alongside get/list/create/delete. Already listed in docs/cli.md “Related APIs”.

Proposed CLI

github-rest-cli repo update -n NAME [-o ORG] [--description TEXT] [--homepage URL] \
  [--public|--private|--internal] [--default-branch BRANCH] [--archived|--unarchived]

Implementation notes

• api.update_repository(...) → PATCH
• parser: repo update subcommand
• tests in tests/test_api.py (+ parser if covered)
• docs/cli.md + CHANGELOG Unreleased
• Auth: repo / Administration (same as create)

Acceptance

• Updates succeed for user and org repos
• Clear errors for 401/404/422
• Docs + tests updated