Prune team member conversation history when team is deleted

Feature Request

When a team is deleted via TeamDelete (or manual cleanup), the terminated teammates' conversation history remains in the session context. This adds noise and consumes tokens for the rest of the session without providing value.

Current Behavior

After TeamDelete or teammate shutdown:

• Team/task directories are cleaned up
• But all teammate idle notifications, shutdown requests/responses, and build output summaries remain in the conversation history
• These messages continue consuming context window space

Desired Behavior

When a team is deleted during a session, automatically prune (or compress) the conversation history related to that team's members — specifically:

• Teammate idle notifications
• Shutdown request/response exchanges
• System teammate_terminated messages
• Optionally: teammate build/task result messages (or at least compress them to a summary)

Why This Matters

In a typical team workflow (spawn 6 build agents, collect results, shut down), the teammate messages can easily add 20-30 conversation turns of protocol overhead (idle notifications, shutdown handshakes, terminated confirmations). After the team is deleted, none of this is actionable — it just burns context tokens.

Suggested Approach

On TeamDelete, trigger the same compression/pruning logic used for context window management, but targeted specifically at messages from the deleted team's members.