Publish `update_mailmap.py` script as GitHub Action

[kdeldycke]

The update_mailmap.py script helps this project keeps track of contributors (and their preferred identities of course).

It has some limitations and edge-cases, but could be made into a stand-alone independent project for others to use as a GitHub Action.

Refs: kdeldycke/mail-deduplicate@2ea15e4#commitcomment-46347705

[kdeldycke]

Analysis: publishing sync-mailmap to the GitHub Marketplace

Current state

The mailmap sync functionality is fully implemented as a gha-utils CLI command (gha_utils/mailmap.py), with unit tests, workflow integration in autofix.yaml, and a PR body template. What's missing is the GitHub Action packaging for Marketplace discovery.

Recommended approach: dedicated action repository

A dedicated repository (e.g., kdeldycke/sync-mailmap-action) with its own action.yml that calls gha-utils under the hood.

Why a separate repo:

• The Marketplace model expects one action per root action.yml. This repo (kdeldycke/workflows) is a reusable-workflows monorepo — adding an action.yml at the root would lock it to a single Marketplace action and shift its identity away from its primary purpose.
• A dedicated repo gets its own versioning, releases, and Marketplace page without interfering with the workflows release cycle.
• Clean separation: this repo stays the canonical source for reusable workflows and the gha-utils CLI; the action repo is a thin wrapper for Marketplace distribution.

What the new repo needs

1. action.yml at the root — composite action that installs and runs gha-utils sync-mailmap. Includes name, description, inputs (source path, create-if-missing flag), outputs, and a branding section (icon + color) for Marketplace display.
2. README.md — usage examples, input/output docs, and a link back to this repo for the underlying implementation.
3. CI workflow — minimal workflow to test the action (e.g., run it against a test repo with known git history and verify the .mailmap output).
4. Versioning — tag releases that pin to a specific gha-utils version on PyPI (e.g., uvx 'gha-utils==X.Y.Z' sync-mailmap), so action consumers get deterministic behavior.

Prerequisites (one-time, on the account)

• Accept the GitHub Marketplace Developer Agreement.
• Ensure two-factor authentication is enabled on the publishing account.

Open questions

• Repo name: sync-mailmap-action, mailmap-action, or something else?
• Scope: Should the action repo expose only sync-mailmap, or also wrap other gha-utils commands as separate actions in subdirectories (only the root action would be listed on the Marketplace, but subdirectory actions could still be referenced directly)?