docs(runbooks): clarify deprecation review signals

[ss-o]

Summary

• Clarifies that quiet or stable repositories are not abandoned by default.
• Requires deprecation review to consider maintainer intent, consumer usage, and replacement availability instead of age or issue volume alone.
• Keeps the existing migration-first sunset flow intact.

Verification

• git diff --check
• ruby -e 'require "yaml"; Dir["workflow-templates/*.yml", ".github/workflows/*.yml", ".github/ISSUE_TEMPLATE/*.yml"].sort.each { |f| YAML.load_file(f); puts f }'
• ruby -e 'abort "missing stability policy" unless File.read("runbooks/deprecation.md").include?("Quiet repositories are not abandoned by default")'

Closes #415

[Copilot]

Pull request overview

Updates the deprecation/sunset runbook to clarify that low-activity (“quiet”) repositories are not presumed abandoned, and that deprecation review should weigh maintainer intent, consumer usage, and replacement availability rather than age or issue volume alone.

Changes:

• Adds explicit guidance that stable/quiet repos trigger review, not automatic deprecation/archival.
• Expands “Step 1 — Decide and record” to require documenting stability intent vs. true unmaintenance, plus replacement availability.
• Keeps the existing migration-first sunset flow intact while refining the decision signals.