Feature: support configurable tag prefix

## Summary
Add an input to configure the tag prefix used by this action (default: `v`).

## Problem
The action currently assumes a `v` prefix (`v1.2.3`) in multiple places. This makes it hard to use in repositories that rely on custom prefixes (for example `pkg-a@1.2.3` in monorepos or org-specific tag schemes).

## Proposal
- Add input: `tag-prefix` (default: `v`)
- Use this prefix consistently when:
- creating tags
- reading/filtering tags
- resolving latest/previous release tag
- setting outputs and release names
- Keep full backward compatibility by defaulting to current behavior

## Acceptance criteria
- Existing users with default config see no behavior change
- Non-`v` prefixes can be used end-to-end without manual workarounds
- Docs and examples include custom-prefix usage
- Test coverage includes both default and custom-prefix scenarios