Prototype: CI validation workflow with blocking and advisory tiers

Part of #12

Question

What does the push/PR validation workflow look like?

Prototype .github/workflows/ci.yml:

Blocking job: XML well-formedness for every addon.xml, python -m py_compile across all .py, and a dry-run of the zip build.
Advisory job: kodi-addon-checker with results in the job summary, continue-on-error so it never blocks.
Establish the baseline: run it once on main and capture how noisy the advisory tier actually is (feeds the "promote to blocking" fog).

Blocked by the artifact-format research (checker invocation details live there).

Deliverable: working workflow on a branch with a green (blocking-tier) run, linked from this issue.

Add prototype two-tier CI workflow

Resolved. Working workflow: .github/workflows/ci.yml on branch JMAN730/setup-matt-pocock-skills, proven by a green run: Blocking checks ✓ / Advisory checker ✓.

Blocking job: addon.xml well-formedness (all addons), python -m py_compile across every .py, and a dry-run of tools/build_repo.py. All pass on today's sources — the repo starts green.
Advisory job: pip-installs kodi-addon-checker (the official Action is stale), derives the addon list from channels.json, runs --branch omega for the Python addons and --branch piers for the skin, strips ANSI, and writes problem/warning counts plus categorized details to the job summary. It exits 0 unconditionally — never blocks.
Measured noise baseline (the number the fog was waiting for): Python addons 12 problems / 257 warnings (245 warnings are a single category: non-whitelisted file endings; problems: xbmc.service schema strictness incl. the start="startup" attribute, mp3streams missing art = Fix plugin.audio.mp3streams broken <assets> paths #19, invalid plugin.program.theredwizard/resources/settings.xml, legacy strings.xml language dirs in mp3streams, wrong fanart size). Skin adds 26 problems / 4 warnings.
Ops note: pushing workflow files required refreshing the GitHub token with the workflow scope (gh auth refresh -s workflow) — done, recorded here for the next contributor who hits the same rejection.

Closing — workflow live and green, see resolution above.