chore: Update warden skills to use getsentry/warden source (#109203)

Move warden and warden-sweep skills from getsentry/skills to
getsentry/warden, their new upstream home. Remove deprecated
sentry-backend-bugs, sentry-javascript-bugs, and warden-lint-judge
skills that are no longer needed.

Also updates .gitignore to ignore the full `.warden/` directory instead
of just `.warden/logs`, and removes `agents.lock` from version control.

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: dcramer

.agents/.gitignore

@@ -19,3 +19,5 @@
 /skills/skill-scanner/
 /skills/sred-project-organizer/
 /skills/sred-work-summary/
+/skills/warden/
+/skills/warden-sweep/