Feature: monorepo/multi-package release support

## Summary
Support releasing multiple package directories in a single repository.

## Dependency
Blocked by #974 (configurable tag prefix), which is needed for package-specific tag naming and parsing.

## Problem
Many users manage multiple Node packages in one repository (`packages/*`). The action currently supports a single `directory` target.

## Proposal
- Add support for multiple package directories (for example `directories` input)
- Release each selected package with deterministic tag naming
- Ensure compatibility with `skip-if-no-diff` semantics per package
- Preserve single-package behavior as the default path

## Acceptance criteria
- Can release more than one package path in one workflow run
- Outputs include package-specific release/tag information
- Backward compatibility for existing single-directory workflows
- Documentation includes monorepo examples and caveats