Add `release branch` command for direct merge-and-tag without release branch lifecycle

[alexrinass]

Add a git flow release branch <version> [<name>] command that merges a branch directly into main and tags it, without creating an intermediate release branch. This was available in git-flow-avh as git flow release branch.

Use Case

When release preparation is already done on develop (or another branch) and there's no need for a dedicated release branch, this command provides a shortcut to go straight to "merge into main and tag."

Proposed Behavior

# Merge develop into main and tag as v2.0.0
git flow release branch 2.0.0

# Merge a specific branch into main and tag
git flow release branch 2.0.0 some-branch

The command would:

1. Merge the source branch (defaults to develop) into main
2. Create a tag with the given version on main
3. Not create, back-merge into develop, or delete any branch

Validations

• Source branch must exist
• Source branch must not be a git-flow prefixed branch (e.g., feature/foo)
• Source branch must not be main itself

Flags

Same tag/merge flags as release finish: --fetch, --sign, --signingkey, --message, --messagefile, --notag, --squash.

git-flow-avh Reference

This mirrors cmd_branch() in git-flow-avh's git-flow-release (lines 763-919).

[alexrinass]

Closing in favor of #26 (promote command). The promote command with --tag support covers this use case more generically — git flow promote develop --tag v2.0.0 achieves the same result without special-casing it on the release command. It also works naturally with the existing base branch parent hierarchy in config.