agent: Add agent.compaction_model setting for context compaction

[enoch-robinson]

Objective

Add a new agent.compaction_model setting that lets users specify a separate language model for context compaction (/compact and auto-compaction), independent of the thread's active conversation model.

Compaction is just summarization — there's no reason to pay Opus prices for it when a cheaper model does the job faster. We've also seen reasoning models misbehave on this task (empty responses, repetition
loops at high effort), so picking a dedicated non-reasoning model for compaction is useful.

Solution

• New compaction_model: Option<LanguageModelSelection> field in AgentSettingsContent and AgentSettings, mirroring thread_summary_model.
• New compaction_model: Option<ConfiguredModel> slot on LanguageModelRegistry with select_/set_/compaction_model() trio, mirroring the existing pattern. The setter deliberately does not emit a registry event in v1; callers read the slot lazily at compaction time.
• New Thread::compaction_model(&self, cx: &App) helper that returns the configured model or falls back to self.model(). Two call sites — Thread::compact and perform_compaction_if_needed — now go through this helper instead of reading self.model() directly.
• build_compaction_telemetry accepts the compaction model explicitly so the model field reflects the model that actually streamed the request. max_tokens still derives from thread.model() (threshold semantics are unchanged).
• Documentation updated at docs/src/ai/agent-settings.md (new user-facing setting).

Example:

{
  "agent": {
    "default_model": {
      "provider": "anthropic",
      "model": "claude-opus-4-6"
    },
    "compaction_model": {
      "provider": "anthropic",
      "model": "claude-sonnet-4-5"
    }
  }
}

Resolution chain:

agent.compaction_model (if set & available)
  → thread.model()  (always available, current behavior)

Behavior change:

Trigger	Before	After
Manual /compact	Uses thread.model()	Uses agent.compaction_model if set & available; else thread.model()
Auto-compaction	Uses thread.model()	Same as above
/compact when thread has no model	NoModelConfiguredError	Succeeds if compaction_model resolves
compaction_model configured but provider missing / model id unknown	n/a	Falls back to thread.model() and logs a one-time warning

Explicit non-goals:

• No Event::CompactionModelChanged(no consumer; the _cx parameter on set_compaction_model is intentionally accepted for future use).
• No GUI selector (consistent with all other feature-specific models).
• No per-profile override.
• No runtime API-error fallback — only config-time failure (provider not registered, model id not in provided_models) triggers fallback. This matches every other feature-specific model.
• No change to threshold calculation, auto-compact trigger, or COMPACTION_PROMPT.
• No propagation to subagent threads.

Testing

3 new unit tests in crates/agent/src/thread.rs::tests:

• test_compaction_uses_configured_compaction_model — manual /compact routes to the configured model; thread's primary model receives no request; telemetry reflects the configured model.
• test_compaction_falls_back_when_compaction_model_unavailable — configured-but-unresolvable falls back to thread.model(); telemetry reflects the fallback model.
• test_auto_compaction_uses_compaction_model — auto-compaction triggered by threshold honors the same setting.

All 14 existing compaction tests still pass. Test suites in crates/agent_settings, crates/language_model, crates/settings_content, crates/agent_ui unchanged. cargo clippy clean on the changed crates.

How reviewers can test:

1. Add agent.compaction_model to settings.json with a cheaper model than the thread's primary model, run /compact, observe the cheaper model receives the request.
2. Set agent.compaction_model to a non-existent provider/model id, run /compact, observe fallback to thread model and a log::warn! line.
3. Trigger auto-compaction by reaching the threshold, observe it uses compaction_model.

Platforms tested: local Linux (cargo check + cargo test on agent / agent_settings / language_model / settings_content / agent_ui crates).

Self-Review Checklist:

• I've reviewed my own diff for quality, security, and reliability
• Unsafe blocks (if any) have justifying comments — no unsafe blocks introduced
• The content adheres to Zed's UI standards — N/A: settings-only change, no UI touched
• Tests cover the new/changed behavior
• Performance impact has been considered and is acceptable — model resolution is an O(1) registry lookup, not on any hot path; no new work in the streaming loop

Release Notes:

• agent: Add support for specifying which model is used for compaction (agent.compaction_model)

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[bennetbo]

Thank you!