chore: add tag + release + validate workflows (no third-party actions)

[kameshsampath]

Context

Add three GitHub Actions workflows to automate tagging, releasing, and PR
validation for the skill repo itself. Same pattern as github-coco-agent — only
actions/checkout@v4 (GitHub-own) + pre-installed gh CLI, no third-party actions.

See github-coco-agent#3
for the full workflow YAML — copy identically to this repo.

Files to add

• .github/workflows/tag.yml — push to main → compute semver bump → push tag
• .github/workflows/release.yml — tag push → gh release create --generate-notes
• .github/workflows/validate.yml — PR conventional commit lint (pure shell)

Version bump logic

Reads conventional commit prefixes from $PREV_TAG..HEAD:

• feat!: / fix!: / BREAKING CHANGE → major
• feat: → minor
• everything else → patch

Use [skip-release] prefix to bypass auto-tag on maintenance commits.

Timing

Add before Jun 16 so the v0.1.0 tag triggers the release automatically.
Coordinate with template repo releases — all three repos tag v0.1.0 on the same day.

Related

• Jun 16 cleanup tracker: chore: Jun 16 — remove PAT + local runner, tag v0.1.0 #12
• github-coco-agent workflows: chore: add tag + release + validate workflows (no third-party actions) github-coco-agent#3
• gitlab-coco-agent workflows: https://gitlab.com/kameshsampath/gitlab-coco-agent/-/work_items/3