[!!!][TASK] Modernize toolchain onto actions/typescript-action template

[stasadev]

The Issue

The action's toolchain was frozen at the state of the upstream fork (GsActions/commit-message-checker) when it went unmaintained: Node 16, ncc bundler, legacy ESLint config, CommonJS Jest, no local-action support. The setup no longer matches the actions/typescript-action template that GitHub maintains as the recommended baseline.

How This PR Solves The Issue

• Bumps runtime to Node 24 (.node-version, action.yml runs.using: node24, nodejs_version: auto in .ddev/config.yaml).
• Switches to ESM end-to-end (package.json "type": "module", .js import suffixes in TS, ts-jest ESM preset with --experimental-vm-modules).
• Replaces ncc with Rollup (rollup.config.ts); dist/index.js regenerated with a source map.
• Migrates ESLint to flat config (eslint.config.mjs); moves Prettier config to YAML (.prettierrc.yml).
• Adopts @github/local-action for local dev (local-action/.env.example, local-action/event.json.example).
• Consolidates Jest mocks into __fixtures__/{core,github}.ts; rewrites tests around jest.unstable_mockModule.
• Marks dist/ as linguist-generated in .gitattributes so GitHub collapses the bundle diff.
• Adds a release workflow that auto-points the major-version tag (v3 to v3.0.x) on every published release.
• Adds AGENTS.md (canonical) with CLAUDE.md and .github/copilot-instructions.md as symlinks so AI assistants share the same project guidance.
• Updates LICENSE to carry both the original Gilbertsoft and the DDEV Foundation copyright lines; preserves "forked from GsActions" credit in README.md and every src/*.ts header.
• Fixes action.yml accessToken which previously defaulted to the literal string "false".
• Refreshes .github/ISSUE_TEMPLATE/bug_report.yml runner list (drops retired Ubuntu 18.04/20.04, Windows 2016/2019, macOS 10.15/11; adds current options and Self-hosted).
• Drops .github/FUNDING.yml, CODE_OF_CONDUCT.md, and .github/ISSUE_TEMPLATE/config.yml (community resources now live under the DDEV Foundation org).

Manual Testing Instructions

• ddev start && ddev npm install
• ddev npm run all (format, lint, test, build) should be clean.
• ddev npm run local-action exercises src/main.ts against local-action/.env and local-action/event.json (auto-copied from the .example files on first run).

Automated Testing Overview

• Existing Jest suite ported to ESM; covers regex pass/fail paths and input/payload error conditions in __tests__/commit-message-checker.test.ts and __tests__/input-helper.test.ts.
• CI (.github/workflows/test.yml) installs, runs npm run all, and fails the PR if dist/ is dirty after a rebuild.

Release/Deployment Notes

• Bumps the required Node runtime from 16 to 24. GitHub-hosted runners have shipped Node 24 since 2025-09; consumers pinning via ddev/commit-message-checker@v3 need no workflow changes.
• The first release after this merge (e.g. v3.0.0) will trigger release.yml, which creates the v3 major-version tag and keeps it current for subsequent patch releases.

[stasadev]

v2.0.0 was never going to work, because we cannot publish it to Marketplace with the same name.

This is going to be v3.0.0, and I'll test it in ddev/ddev.