Consolidate CI into ci.yml (no pyproject; docs stay npx)

[coretl]

Goal

Consolidate GitHub Actions CI into a single ci.yml. No pyproject.toml — meta-panda is a Yocto/bitbake layer with no Python test harness (only a handful of recipe helper scripts) and pure-MyST docs, so there are no central Python dependencies worth tracking. Work happens on a branch off docs and merges back into docs.

CI consolidation

• Fold code.yml (Code CI: _make_boot/_release) into the single existing ci.yml, as jobs calling the reusable _*.yml workflows. Docs (_docs.yml/_publish.yml/_make_image/_make_sdk) stays as-is.
• make stays the runner — CI just orchestrates.
• No ruff/lint job (no pyproject.toml to configure it against).

Docs

• Keep npx for mystmd (pinned via MYSTMD_VERSION in CONFIG). meta-panda's docs are pure MyST with no Python, so there is nothing to gain from pip-via-uv.

Scope note

Sibling repos PandABlocks-FPGA and PandABlocks-server additionally get a minimal uv-managed pyproject.toml; meta-panda is intentionally CI/docs-only.

Checklist

• Branch off docs
• Fold code.yml into ci.yml
• Verify CI green; PR back into docs