feat: add ignore-bot-updates option

[neenhouse]

Description:

Adds an ignore-bot-updates option (default false). When enabled, activity caused by bot accounts (user type Bot) does not remove the stale label and does not block closing. Human activity behaves exactly as before: a comment, a label change, or (for pull requests) a pushed commit removes the stale label and restarts the cycle.

Motivation. In repositories with heavy automation — label-sync workflows, preview deployments, generated sticky comments — every bot touch bumps updated_at, removes the stale label, and restarts the full days-before-stale countdown. The stale cycle never completes: items get warned, un-staled by a bot within a day or two, re-warned two weeks later, forever. In one repo we measured, ~100 PRs sat in this orbit while only a handful ever closed. The comment filter in _hasCommentsSince already ignores bot comments, but the updated_at check (issueHasUpdateSinceStale) and the close-window check both count bot activity, so bot-heavy repos can't converge.

Behavior details:

• The un-stale check attributes updates via issue events: if every event since the stale marking was caused by a bot (or was the stale labeling itself — extending the existing hasOnlyStaleLabelingEventsSince logic), the update does not un-stale the item.
• For pull requests, the last commit date is checked so a human push — which bumps updated_at without leaving an issue event — still counts as human activity.
• An updated_at bump with no attributable cause (e.g. a body edit, which leaves no issue event) is treated as human activity. The fail-safe direction is keeping items open.
• The close-window check runs from the stale-marking date instead of updated_at, since bots keep updated_at perpetually fresh.
• IIssueEvent gains an optional actor field (already present in the REST payload the action fetches; previously just untyped).

Backward compatibility: with the option disabled (default), behavior is unchanged — the disabled path is covered by a regression test that mirrors the enabled-path fixture and asserts the old outcome.

Related issue:

Closes #1243 (option to exclude some update types from the last-update calculation); related: #795 (bot comment counted as regular update — the comment half was fixed, the updated_at half wasn't), #1309 (narrower fix: bot comments only; this PR also covers bot label/events churn and protects human pushes on PRs).

Check list:

• Mark if documentation changes are required. (README: options table row + #### ignore-bot-updates section)
• Mark if tests were added or updated to cover the changes. (__tests__/ignore-bot-updates.spec.ts, 5 cases; full suite 1365 passing)