skills: codify GCP authoring lessons from CodeBundle live-run reviews

[theyashl]

Adds transferable authoring guidance distilled from maintainer review of shipped GCP CodeBundles — bundles that passed the structural scorer but failed on the first live run. Kept general and cloud-agnostic; deliberately no bundle-specific recipes (an earlier draft included a bespoke monitoring_query.sh helper + a specific-command ban — removed as too task-specific for a reference doc).

auth-gcp.md

• New "Cross-Project Quota Project" section — set CLOUDSDK_BILLING_QUOTA_PROJECT=${GCP_PROJECT_ID} in the suite env. Without it, a cross-project SA / Workload-Identity run derives the quota project from the credential and fails SERVICE_DISABLED against the caller's project. No-op for in-project SAs, so safe to always include.
• Refined the "degrade gracefully" shell rule — only swallow errors (2>/dev/null || echo "[]") from commands that exist and can only fail on permissions; for calls that can fail otherwise, validate the response shape and fail loud instead of defaulting to [] (a swallowed error reads as "no data" → scored healthy).

sli-authoring.md

• "Reading Metrics Safely" — don't swallow command-not-found / API errors into empty data; prefer a first-class, tested call over a guessed subcommand/flag.
• "Report Enrichment, Not Rollup Tasks" — don't add a redundant summary task that re-queries what the per-dimension checks already collect; the SLI aggregates natively, so enrich each check's report instead.

test-infra-gcp.md

• Loading test data after provisioning for data-dependent checks (null_resource + local-exec, batched/encoded payloads), plus a note on provider metric lag/flooring.

skill-template-authoring.md

• Make regenerating SKILL-TEMPLATE.md via generate_skill_md.py a required PR step; note the generator's description-truncation / sourced-helper caveats.

Provenance

These lessons come from the review of the gcp-cloudspanner-* CodeBundles (rw-cli-codecollection #725–#727). The corresponding fixes are already applied in those bundles; this PR turns the review follow-ups into permanent authoring guidance the Creator/bot reads.

🤖 Generated with Claude Code