[feat] Add branch creation CLI and PR branch-name validation

[ThreeLight2000]

Motivation

Provide a product-neutral branch naming workflow that creates valid branch names before work begins and catches custom branch creation paths before merge.

Scope

• Add a CLI command for feat, fix, refactor, docs, and chore branches.
• Document the command and naming contract in AGENTS.md and CONTRIBUTING.md.
• Add a GitHub Actions check for pull request source branch names.

Acceptance criteria

• The CLI creates branches matching /-.
• Invalid types, issue numbers, and slugs fail before branch creation.
• The pull request check fails for invalid source branch names.
• Existing branch creation paths remain possible but cannot merge without a passing check.

Verification plan

• Run CLI success and failure cases.
• Run the repository verification command.
• Validate the workflow syntax and regex.

[ThreeLight2000]

Implementation started directly on main as requested. Added the branch creation CLI, repository documentation, and PR branch-name validation workflow. Verification: pnpm verify passed; CLI help and invalid-input cases passed; isolated branch creation test produced feat/123-add-sso.

[ThreeLight2000]

Implementation and verification complete in the main working tree. The Issue remains open for review because changes are not committed or pushed. pnpm verify, workflow YAML parsing, CLI help/invalid-input checks, and isolated branch creation all passed.