Add configurable token usage threshold alert for agents #44208

Feature hasn't been suggested before.

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Describe the enhancement you want to request

Problem
When a session approaches the token limit, compaction kicks in silently and older context gets summarized. By the time the user notices degraded quality, they've already lost detail from earlier in the conversation. There's no way to proactively know when to start a new session.
Proposed solution
Add a configurable compaction.alert option (e.g. 0.8 for 80%) that triggers a visible warning to the user when token usage crosses the threshold. This could be:

• A TUI notification/banner
• An agent-injected message in the conversation
• A configurable action (e.g. "warn", "auto-compact", "notify")
  Config example
  {
  "compaction": {
  "auto": true,
  "alert": 0.8
  }
  }
  Use case
  Users working on long sessions (debugging, planning, large refactors) would benefit from knowing when to start a fresh chat to maintain conversation quality, rather than discovering context loss after the fact.