Add CONTRIBUTING.md with lessons from the Cloud Run work

[harish-narayanappa-10]

Summary

Adds a contributor guide based on the review rounds for the GCP Cloud Run PRs, so the next person changing the skill or adding a compute provider can avoid the same back-and-forth. Most of those comments were about how to write skill instructions, not about Cloud Run itself, so the guide applies to every change.

• CONTRIBUTING.md (new)::
  • Writing durable instructions: no authoring narration, test-run timings,
    harness-specific wording or liudes a before/after table ofreal rewrites.
  • **Correctness and safety:*, region-scoped queries,checking parsed output against the real CLI, keeping secrets out of the agent, and
    designing diagnostics around e
  • Validation and review: running SDK guides as written, sentence-level review,
    fixing a flagged pattern everyked PRs.
  • Adding a compute provider: the layout to extend, provider-neutral shared files,
    one source for shared mechaniclast.

The guide ends with two checklists ordered by impact: one for every change, one for a new provider.