docs: add runner OpenAPI schema fetch brittleness + --validate=false guide

[weicao]

Summary

New methodology doc addon-runner-openapi-schema-fetch-brittleness-guide.md covering when test harness kubectl apply calls hit transient failed to download openapi: ... Client.Timeout / context cancellation errors against unstable remote API entry, and how to scope --validate=false defensively.

Body (generic methodology, version-agnostic, no engine binding):

• §1: Why kubectl apply default client-side OpenAPI schema fetch is a brittle path; 3 distinguishable failure signatures
• §2: 5 hard rules — server-side admission must NOT be relaxed; --validate=false only on identified brittle apply points (never global alias); change is runner-harness hardening (NOT product / addon / KB fix evidence); 3-class apply failure classification (server admission / RBAC / network transient); consolidate brittle apply points into helper for single grep coverage
• §3: patch-gate 3 indicators
• §4: 5-point PR review checklist
• §5: 2 anti-pattern vs correct-pattern pairs
• §6: relation to addon-runner-incluster-vcluster-access-pattern-guide.md, addon-test-acceptance-and-first-blocker-guide.md, addon-evidence-discipline-guide.md, addon-postready-bounded-timeout-failure-classification-guide.md

Appendix A is OceanBase enterprise addon T-PITR.4 OpenAPI body read cancellation case (RUN_ID pitr-runtime-postreadyfix-N2-...) + runner-side helper landing on tests/pitr.sh sha 6edf74fc... → f4b3375e... + structural follow-up via addon-runner-incluster-vcluster-access-pattern-guide.md (in-cluster DNS removes the workstation hop). Explicit boundary: helper landing only proves apply succeeded in 1 subsequent sample; NOT release-ready / NOT permanent immunity.

SKILL-INDEX.md updated: added entry under ### 5. 改造 runner / 工具链.

Test plan

• Manual: methodology body has zero OB-specific commands
• Manual: appendix is the only OB-specific section and has explicit boundary
• Manual: cross-doc references resolve

🤖 Generated with Claude Code

[weicao]

Blocking for merge:

1. PR body still contains 🤖 Generated with [Claude Code].... Public PR body must not include AI/tool attribution.
2. The new guide intro has only 4 standard fields. Please add > **Affected by version skew**: ... after Applies to KB version.
3. This PR links to docs that are not on main yet: addon-runner-incluster-vcluster-access-pattern-guide.md (docs: add runner in-cluster vcluster access pattern guide #124) and addon-postready-bounded-timeout-failure-classification-guide.md (docs: add postReady bounded timeout + failure classification guide #122). Please merge/rebase in dependency order, or remove/defer those links until targets exist.

One content note for the recheck: keep --validate=false framed as runner-harness hardening only; it must not be described as product/addon/KB stability evidence.

[weicao]

Closing per Allen's docs PR queue triage (DM 2026-05-18). Superseded by main-branch standard guide(s).