feat(smith): OpenRouter as a first-class provider and route target

[edwin-zvs]

What

Adds OpenRouter (https://openrouter.ai) as a first-class model provider, following the DeepSeek/Grok pattern: a thin variant of the existing OpenAI chat-completions client plus a built-in route target. Motivated by stealth models like stealth/ox-alpha (1M context, tool calling, currently free) that appear on OpenRouter before they have official SDKs — with this, any new routed model is reachable the day it drops via /model openrouter:vendor/model, no per-model code.

• openrouter: model prefix in smith's spec routing. Explicit only — no bare-name sniff: OpenRouter ids are vendor/model slash paths, which collide with Ollama namespaces (hf.co/user/model), and an aggregator is a distinct billing path (spec 0028). Variant suffixes (:free, :nitro) pass through in the model id.
• OpenRouter mode on the OpenAI client (OpenAi::openrouter): sends OpenRouter's usage: {include: true} accounting knob and reads the response's cost field, so Usage.usd carries the generation's actual USD cost for any routed model — no price table. Also sends the optional HTTP-Referer / X-Title attribution headers.
• Built-in route target (spec 0179): OPENROUTER_API_KEY alone makes openrouter a route target (default model openrouter/auto, the aggregator's meta-router id). provider_dialect maps it to OpenAI-chat, so native harnesses can be routed through it like any other openai-compatible target. A declared [smith.models.openrouter] profile still overrides the built-in.
• Auto-detect ladder: OPENROUTER_API_KEY is the new last rung (yields to every direct vendor key), for both sessions and title-gen.
• Context budgeting: 128K floor for openrouter (heterogeneous models; overflow learn-and-retry tightens per model). Effort support stays unadvertised until a routed model is measured to grade it (spec 0160's model-aware rule).
• Surface updates: /model completions (openrouter:openrouter/auto, openrouter:stealth/ox-alpha), /configure smith-auth method + guidance, config.toml template comments, credential-error hints.
• Spec 0208 records the decision (explicit aggregator route, wire-reported cost, meta-router default).

Testing

• cargo build + cargo test --workspace: all green (1455 daemon, 748 protocol, 229 smith, …, 0 failures).
• New tests: prefix parsing incl. slash ids and :free suffixes, bare-slash-id does NOT route to OpenRouter, auto-detect precedence with the new rung, profile endpoint/key defaults; the existing every_builtin_target_is_routable_and_self_consistent guard covers the new built-in.

No TUI-visible rendering change, so no recording — the new surface is model specs, route targets, and the /configure list.

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