docs(contributing): fleet suites must move with generator eligibility changes

[joshua-temple]

Problem

A change to the generator or CLI that alters which environments are eligible for an operation (for example, PR #418 making the first environment ineligible for rollback) can land without a matching update to the fleet example-repo suites that exercise that operation. The mismatch stays latent, since the affected fleet lane simply skips or no-ops, and only surfaces later as an apparently unrelated fleet failure on an unrelated change.

Fix

Add an explicit standard to the "Making a change" checklist in CONTRIBUTING.md: when a change alters the eligibility or validity of an operation on an environment, the fleet example-repo suites exercising that operation must be updated in the same change. A generator or CLI eligibility rule and the fleet suite asserting against it are treated as one coupled unit.

Verification

• Diff reviewed for the repo's OSS-hygiene guardrails (no banned terms, no em dashes, no hyphen-as-dash usage); clean.
• Checked docs/src/content/docs/ for a mirrored contributing or standards page; none exists (only incidental hits on the unrelated contributors manifest field), so no docs UI change is needed.
• Markdown-only change; no build/lint/test impact.