* feat(extensions): add full Custom Agent team collaboration plugin
* fix(extensions): register agent teams in the capability catalog
* fix(agent-teams): recover incomplete teams and preserve request capacity
* fix(agent-teams): preserve clarification waits and active handoff lineage
* fix(agent-teams): align mention and input text limits
* feat(frontend): support extension providers in composer mentions
* fix(mentions): retain settled plugin suggestions during search
* fix(extensions): validate identifiers and skip empty mention searches
* test: stabilize history pagination after latest page refresh
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Co-authored-by: Willem Jiang <willem.jiang@gmail.com>