ci: base stale-assignment clock on human activity, not updated_at

[andreahlert]

Problem

The Stale Assignments workflow (.github/workflows/stale-assignments.yml) is supposed to warn at 14 days of inactivity and unassign at 21. It never unassigns.

It measures inactivity from issue.updated_at:

const updatedAt = new Date(issue.updated_at);
const daysSinceUpdate = Math.floor((now - updatedAt) / (1000 * 60 * 60 * 24));

updated_at is bumped by any change to the issue, including the bot's own warning comment and label edits. So each weekly warn resets the clock, capping daysSinceUpdate at the warn interval. The 21-day unassign branch is effectively dead code: the bot warns forever and never removes an assignee.

Evidence (#400)

Bot warning timestamps on #400:

Date	Text
2026-04-15	inactive for 17 days
2026-04-29	inactive for 14 days
2026-06-03	inactive for 16 days
2026-06-24	inactive for 20 days

Apr 15 to Apr 29 is exactly 14 days. An honest counter would have read 31, not 14, the warning comment reset updated_at. The assignee was never removed despite four warnings.

Fix

Measure inactivity from the last human activity, taking the most recent of:

• last non-bot comment (github-actions[bot] excluded),
• the assigned timeline event for a current assignee (so a freshly assigned issue is not instantly stale),
• issue creation time (fallback).

Bot comments no longer pollute the signal, so the clock accumulates past 14 and the 21-day unassign threshold becomes reachable. The paginated comments are reused for the existing recent-warn spam guard.

Notes

• No behavior change to thresholds (14 warn / 21 unassign) or to the lifecycle/frozen exemption.
• Security note in the header still holds: only numeric IDs and login names from the API are used; no untrusted input reaches a shell.
• Validated locally: YAML parses, embedded script passes node --check.

[andreahlert]

Superseding this. Rather than chase a correct activity signal, removing the stale-assignment bot entirely (it posted misleading inactivity numbers and never actually unassigned anyone). New PR removes the workflow.

[andreahlert]

Reopened. Disregard the previous close note about removing the workflow, that was a misread. The bot stays; this PR is the intended fix: the bot's own warning comment no longer resets the inactivity clock. Excluding github-actions[bot] comments from the activity signal is the whole point, so warnings accumulate and the 21-day unassign threshold becomes reachable.