docs: add netlify-build-failure runbook to README Operational Readiness index

[kubestellar-hive]

What

Adds runbooks/netlify-build-failure.md to the README's ## Operational Readiness index, next to deploy-rollback.md.

Why

Fixes #6794. The runbook was added in #6782 (and is already linked from runbooks/deploy-rollback.md), but was never added to the README's index of runbooks, so it was undiscoverable to anyone scanning the README for available runbooks during an incident (e.g. a direct-to-main Netlify build failure alerted by netlify-error-reporter.yml).

Scope check

• Documentation-only change (one README bullet).
• Does not touch any workflow file.
• Does not weaken any existing alert or SLO.
• Does not add a probe.

Closes #6794.

— hive: agent=operations backend=copilot model=claude-sonnet-4-6

[kubestellar-hive]

Important

Held for human sign-off on the direction, not on the code.

This PR's only tracked rationale is #6794, which the hive filed itself — issue #6794 was filed by kubestellar-hive[bot] and no human has acknowledged it. An agent-filed issue does not, on its own, establish that anyone agreed to the direction (hivecommons/hive#5117).

The change may well be right; nothing here is a review of it. To release the hold, acknowledge the direction on that issue — comment on it, assign yourself, or add the approved-direction label — and remove the hold label here.

[netlify]

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[kubestellar-prow]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[kubestellar-prow]

Hi @kubestellar-hive[bot]. Thanks for your PR.

I'm waiting for a kubestellar member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

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[clubanderson]

Direction acknowledged by hub admin (approved-direction added on #6794); releasing the #5117 hold. This hive runs at ACMM L6 and may act on its own findings.

[github-actions]

Thank you for your contribution! Your PR has been merged.

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