docs: add cluster Running gate budget sizing guide

[weicao]

Summary

New methodology doc addon-cluster-running-gate-budget-guide.md covering how to size test-runner T1 wait_cluster_running budget based on env P99 (not product normal start time), and how to classify the failure layer when the budget is exceeded.

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

• §1: Anti-patterns (using product normal time / using arbitrary huge number)
• §2: 5 hard rules — env-variable budget, P99+50% default from env sample distribution, 4 observation outputs on timeout, generic exit reason for upper-layer classification, 3-level budget nesting (caller > helper > kubectl --request-timeout)
• §2 Rule C: 7-row layer classification hint table
• §2 Rule D: "exceeded budget is NOT product RED until classified"
• §3: 6-point PR review checklist
• §4: 2 anti-pattern vs correct-pattern pairs
• §5: relation to addon-bounded-eventual-convergence-guide.md, addon-postready-bounded-timeout-failure-classification-guide.md, addon-test-acceptance-and-first-blocker-guide.md, addon-evidence-discipline-guide.md

Appendix A is OceanBase enterprise addon postreadyfix3 (1200s false-positive due to env-slow-start cold pull) → postreadyfix4 (1800s default after env P99+50% calibration) case. Explicit boundary: 1800s default only calibrated for idc4 vcluster + apecloud registry + apelocal-hostpath SC scope; other env must independently calibrate. 6 subsequent samples T1 <= 4min, NOT product validation / NOT release-ready.

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 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 addon-postready-bounded-timeout-failure-classification-guide.md from PR docs: add postReady bounded timeout + failure classification guide #122, which is not on main yet. Either merge/rebase after docs: add postReady bounded timeout + failure classification guide #122 lands, or mark the cross-link as pending and keep this PR blocked until the target exists.

Content framing is otherwise in the right shape: budget by env distribution, classify before product RED, and appendix scope stays narrow.