feat: add TAG_FROM_BRANCH to derive the tag from the release branch name

[postalservice14]

Summary of changes

Adds an opt-in TAG_FROM_BRANCH option (default false) that takes the tag verbatim from the release version embedded in the branch name, instead of bumping via #major/#minor/#patch tokens or DEFAULT_BUMP.

• Reads the version from the merge/squash commit message first, then falls back to the current branch name (covers both merge-to-main and running directly on a release/X.Y.Z branch).
• New BRANCH_VERSION_REGEX (default release[/-]([0-9]+\.[0-9]+\.[0-9]+)) makes the pattern configurable; it must contain exactly one capture group for the X.Y.Z. The WITH_V/TAG_PREFIX prefix is applied to the captured version.
• If no version can be recovered (e.g. a rebase-merge dropped the branch name) the action fails loudly rather than producing a wrong tag.
• CUSTOM_TAG keeps precedence if both are set.
• Matching uses bash [[ =~ ]] / BASH_REMATCH, so untrusted commit text is never interpolated into a shell command.

Both options are documented in the README Options section.

Closes #359

Breaking Changes

Do any of the included changes break current behaviour or configuration?

(YES / NO) — the feature is gated behind a default-false flag and slots in before the existing case "$log" bump path, so every current workflow behaves identically. No outputs changed.

How changes have been tested

• bats test/test_branch_tag.bats test/test_prefix.bats → 13/13 pass (6 new + 7 existing).
• New tests cover: merge-commit message, squash-merge message, current-branch fallback, existing prefixed tag as base, the loud-failure rebase guard, and a custom BRANCH_VERSION_REGEX.
• shellcheck entrypoint.sh — clean on the diff.

List any unknowns

• Whether hotfix/* (and similar) should be matched by the default regex, or left for users to set via BRANCH_VERSION_REGEX. Current default targets release/ and release- only.