Add repository deprecation runbook

[ss-o]

Summary

Add a deprecation runbook for quiet or obsolete repositories so maintainers can distinguish stable-done projects from projects that should be archived, redirected, or explicitly retired.

Scope

• define signals for deprecation review using issue activity, maintainer intent, and replacement availability, not last commit date alone
• document communication steps before archive or retirement
• include README notice, issue/PR handling, tracker updates, and replacement links
• require ADR or maintainer decision for non-obvious retirements

Acceptance criteria

• quiet stable plugins are not treated as abandoned solely because of age
• deprecation steps are explicit enough for an agent or maintainer to follow without inventing policy
• runbook links to tracker and handoff guidance

Completed

Completed by runbooks/deprecation.md, merged in #433.

Evidence:

• defines replacement and maintainer-intent based review signals rather than age-only archival
• requires a tracker issue and ADR for non-obvious or cross-repo decisions
• documents README and wiki notices, migration steps, transition windows, tracker updates, catalog cleanup, and archive behavior
• explicitly preserves published artifacts and existing install paths during transition
• links release and triage runbooks