Budget-aware multi-model routing with per-task cost tracking

Summary

Add per-task cost tracking with fallback chains and hard spending limits to LLM routing. Each agent operation gets a budget allocation; if the primary model is too expensive or unavailable, fall back to cheaper alternatives automatically.

Motivation

Inspired by Ouroboros per-round cost tracking with per-worker budget allocation. Current state:

• terraphim-llm-proxy routes between multiple models but has no per-task budget tracking
• genai (terraphim fork) handles LLM calls but does not meter usage or cost
• The multi-model judge pattern (quick -> deep -> tiebreaker) needs budget awareness: only escalate to expensive models when cheaper ones disagree
• No spending limits exist -- a runaway agent loop could consume unbounded API credits

Implementation Plan

1. Budget Struct

pub struct TaskBudget {
    pub max_cost_usd: f64,
    pub model_chain: Vec<ModelConfig>,  // ordered by preference (cheapest first)
    pub spent_usd: f64,
    pub request_count: u32,
    pub created_at: jiff::Timestamp,
}

pub struct ModelConfig {
    pub model_id: String,
    pub cost_per_1k_input: f64,
    pub cost_per_1k_output: f64,
    pub max_latency_ms: u64,
    pub fallback_on: Vec<FallbackReason>,  // Timeout, RateLimit, BudgetExceeded, EmptyResponse
}

2. Cost Tracking in terraphim_service

• After each LLM call, compute cost from token counts + model pricing
• Accumulate in TaskBudget.spent_usd
• If spent_usd >= max_cost_usd, reject further calls for this task
• Log cost events to event store (#597)

3. Fallback Chain Logic

• Try models in model_chain order
• On failure (timeout, rate limit, empty response, budget exceeded for this model tier), try next model
• If all models exhausted, return error with cost summary

4. Integration with terraphim-llm-proxy

• Proxy already reports token counts in responses
• Add cost metadata to proxy response headers (cost_usd, model_used, tokens_in, tokens_out)
• Agent-side budget tracking consumes these headers

5. MCP Resource Exposure

• Expose /budget/current as MCP resource showing active task budgets
• Expose /budget/history for cost analysis over time

Affected Crates

• terraphim_service (primary -- add budget tracking to LLM routing)
• terraphim_types (add TaskBudget, ModelConfig types)
• terraphim_multi_agent (wire budget into agent task execution)
• terraphim_mcp_server (expose budget resources)

Dependencies

• #597 Event sourcing (for cost event persistence)

Estimated Effort

~4 hours (mostly plumbing between terraphim_service and proxy response headers)

Part of

Epic #595

Start in the affected terraphim_service, terraphim_types, terraphim_multi_agent, and terraphim_mcp_server crates, then review the event-sourcing dependency in #597 and the existing terraphim-llm-proxy token headers. Done means task budgets meter costs, enforce limits, fall back through model chains, persist cost events, and expose the current and historical MCP budget resources.