branch-name: allow overriding prefix list without rewriting full regex

Follow-up from review of #25 (checks/branch_name.sh).

Today the branch convention is controlled only by the full `branch-pattern` regex (default `^(feature|bugfix|hotfix|release|support|chore|docs|ci|dependabot)/[a-zA-Z0-9._-]+$`). Adding or removing a single prefix forces the user to copy and re-author the entire regex.

Proposal: add a granular input (e.g. `branch-prefixes`, comma-separated) that overrides just the allowed prefix list while keeping the rest of the pattern intact. Composable with `branch-require-ticket`.

- Keep `branch-pattern` as the full-override escape hatch (back-compat).
- When `branch-prefixes` is set, build the regex from it.
- Document precedence between `branch-prefixes` and `branch-pattern`.

Ref: review comment by @miroslavpojer on checks/branch_name.sh.