feat(skills): Add AXIS skill evals

[dcramer]

This standardizes skill evaluation on AXIS, using its built-in Codex adapter so evals run through the real codex exec --json harness instead of custom runner scripts.

The new maintainer path for skill-writer is:

npx @netlify/axis run --config skills/skill-writer/evals/axis.config.json
npx @netlify/axis run --config skills/skill-writer/evals/axis.config.json --scenario small-inline-workflow
npx @netlify/axis reports latest --config skills/skill-writer/evals/axis.config.json --html

Each AXIS scenario copies the repo into an isolated temp workspace, excludes local agent/report state, installs the local skill under test with skills: ["./.."], runs Codex, and captures transcript/judge/artifact evidence. Generated .axis/ report directories are now ignored so local eval runs do not pollute future diffs.

The initial skill-writer evals cover three behavior classes:

• small-inline-workflow: catches simple skills being overbuilt with references, scripts, or broad source research.
• reference-backed-integration: checks that complex/integration skills keep SKILL.md as a router and split optional depth into focused references.
• iteration-from-bad-output: checks that skill-writer can improve an existing bloated skill by narrowing triggers and removing unnecessary files.

The runtime references/skill-evals.md teaches future skills the same pattern: add EVAL.md, evals/axis.config.json, and focused AXIS scenarios with observable judge checks. It explicitly keeps AXIS as the prescribed skill-eval framework and treats Promptfoo/custom Codex scripts as out of scope for this repo's skill evals.

pr-writer gets a proof eval focused on actual behavior rather than eval plumbing:

npx @netlify/axis run --config skills/pr-writer/evals/axis.config.json --scenario concise-docs-pr

That scenario creates a docs-only skill change and verifies the generated PR title/body stay concise, use an appropriate conventional title, avoid template headings, and omit validation/tool boilerplate. The renamed concise-docs-pr case is meant to be a pattern for behavior-focused eval cases, not framework smoke tests.

I exercised the new paths locally: small-inline-workflow completed through AXIS/Codex with score 94, and concise-docs-pr completed with score 99. Structural validation also passed for both skill-writer and pr-writer.