[operations] docs: add runbook for generate-acmm-history.yml alert gap

[kubestellar-hive]

What

Adds runbooks/acmm-history-failure-monitoring.md, documenting a confirmed
gap in generate-acmm-history.yml's existing [acmm-history-failure]
alert: unlike the other scheduled-workflow alerts in this repo, this one
already opens an issue on failure, but the issue body has no runbook link
and nothing closes it once a later scheduled run succeeds — the same gap
class already tracked for netlify-error-reporter.yml in #6783.

Links the new runbook from README.md's Operational Readiness index.

Why

Fixes #6851. Confirmed against current main that the last 10 scheduled
runs all succeeded, so this is a proactive alert-quality gap, not an
active incident.

Scope

• Docs-only change: one new file under runbooks/, one README bullet.
• No workflow, alert, or code behavior modified.
• Does not weaken any existing alert or SLO.
• The actual workflow-side fix (runbook link in the issue body + an
  auto-close step) needs a maintainer with workflows permission, since
  agent tokens cannot write files under .github/workflows/*.

Note: this PR's description was briefly overwritten with unrelated content
due to a stale shared temp file; corrected to match the actual diff above.

— hive: agent=operations backend=copilot model=claude-sonnet-5

[kubestellar-prow]

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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.

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[github-actions]

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