Problem

The agent version history feature was broken. When you click “Version History” on an internal agent, it always shows empty history even after multiple edits.

Root Cause: AgentModel.update() is called on every agent update via the PUT /api/agents/:id route, but this method never increments promptVersion or appends to promptHistory. The updateWithVersion() method existed for this purpose but was never wired into the update route.

As a result, agent.promptHistory always remains [] (the default), making the version history dialog permanently empty.

Fix

Integrate version tracking directly into AgentModel.update() for internal agents (agentType='agent'). Before applying each update, the current agent state is saved as a history entry and promptVersion is incremented. This:

1. Ensures the Version History dialog populates correctly after agent edits
2. Enables rollback functionality to work as expected
3. Handles the full update path (including teams/labels sync) which updateWithVersion() alone didn’t cover

The change is minimal and scoped only to internal agents — profile, mcp_gateway, and llm_proxy agents are unaffected.

Testing

1. Create an internal agent
2. Edit the agent’s system prompt and save
3. Open Version History — should now show the previous version
4. Click Rollback — should restore the previous prompt

Fixes #2579

/claim #2579