chore: move stale-commit from hard rule to soft signal

After auditing the existing list against the original draft of rules, we
found that a hard ">24 months since last commit" rule would disqualify
~1,500 repos — many of which are stable, widely-used "complete"
libraries (Masonry, PureLayout, MagicalRecord, GPU Image). Inactivity
alone is a poor proxy for quality.

Moves "commit in the last 24 months" from the hard rules to the soft
signals in both CONTRIBUTING.md and the PR template, and adjusts
deletion criteria accordingly. Prolonged inactivity can still be a
reason for removal through periodic audits or community-proposed
issues/PRs, but is no longer an automatic trigger.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>