Brain: route managed Claude Code sessions to cheaper models (Haiku-for-obvious)

[mercurialsolo]

Distinct from #370

#370 (shipped) routes the brain's own local-LLM usage (cheap triage → strong on ambiguity). This issue is the #240 angle: routing the managed Claude Code sessions' model — run obvious/bulk turns on a cheaper model (Haiku) and reserve Opus for hard turns.

Strong market validation (see #240): the r/ClaudeAI "$0.02/call coworker" thread (1.77K upvotes) is people DIY-ing exactly this tiering by hand.

Mechanism

• The rules engine (rules.rs) already has a route action; the missing piece is a model-selection action + a brain confidence/complexity signal that triggers it.
• Brain decides "this turn is obvious bulk work → suggest tier-down" vs "ambiguous/risky → keep the strong model."
• Pairs with the budget-ETA forecast (Model routing + cost forecasting — proactive cost control #370 inc 1): when the conservative ETA bound nears budget, the suggested action is "tier down for the rest of the window."

Scope notes

• Needs a way to actually change a live session's model (or surface the recommendation to the operator) — investigate what Claude Code exposes.
• Start advisory (recommend in the TUI / plugin) before auto-applying.

Cross-ref: #240 (original), #370 (brain-internal routing, shipped), epic #367.