chore: define stale PR policy and clean up inactive PRs

[andreahlert]

Part of #690

Establish a stale PR policy and apply it to the current backlog.

Policy

• PRs with review comments and no author response for 14 days → pr/stale → convert to draft
• PRs with no activity for 90 days → lifecycle/stale → close with comment inviting reopen
• Umbrella/roadmap PRs can be marked lifecycle/frozen to exempt from auto-close

Current backlog to address

Draft PRs from elijahbenizzy (2024):
#49, #269, #330, #365, #444, #519, #533

Stale contributor PRs (no recent activity):
#143 (WIP for tools, Jun 2024), #267 (Instructor+Gemini, Jul 2024), #291 (debug endpoints, Jul 2024), #401 (favicon, Oct 2024)

Dependabot PRs (untouched for months):
#566, #568, #582, #583, #584, #594, #597, #606, #626, #627, #636, #642, #657, #658, #660

Recommendation

Batch-merge or close the dependabot PRs. For stale contributor PRs, post a comment asking if the author still wants to continue, then convert to draft or close after 14 days with no response.