Adding a model provider

The whole point of the abstraction: adding a provider must not touch the agent loop, the router, or
the UI. If it does, the abstraction leaked and that is the bug to fix first.

Contract

Implement ModelProvider (PRD §79): id(), models(), stream(), health(), capabilities().
Provider-specific request shapes, error codes, token field names and streaming framing stay inside
the adapter. What crosses the boundary is the normalised ModelRequest/ModelStream and a
normalised error.

Required before merge

Every adapter passes the same contract suite. No provider gets an exemption.

• Authentication — each declared authModes path actually works
• Model discovery — returns real models from the account, never a hardcoded list
• Streaming — tokens arrive incrementally, not buffered and released at the end
• Cancellation — dropping the stream stops the upstream request
• Tool calls — round-trip through the normalised tool schema
• Usage parsing — input, output, cached and reasoning tokens mapped to UsageEvent
• Error normalisation — auth, rate limit, context overflow, server error, timeout each map to a
  distinct normalised variant the router can act on
• Rate limit handling — 429 with retry-after respected
• Timeout — bounded, configurable
• Fallback — the router can move the task to another provider without losing state

Rules

• Never invent numbers. If the provider does not expose remaining quota, cost or a token count,
  report it as unknown. A plausible guess in a spend dashboard is a lie with a dollar sign on it.
• Never reverse-engineer authentication. No scraping cookies, browser sessions, private OAuth
  tokens, or another product's credential files. API keys, documented OAuth, enterprise credentials
  and local endpoints only. (PRD §18–20)
• Never hardcode key formats. Providers change them; validate by using the key, not by regex.
• Emit a usage event per request, at the adapter, including on failure.
• Secrets come from the vault, are never logged, and never appear in a prompt or an error
  message. Add a redaction test.

Capabilities are declared, not assumed

ProviderManifest declares streaming, tools, vision, reasoning, usage API and cost API support.
The router reads these. An adapter that overstates its capabilities breaks routing for everyone.

Local providers

Ollama, LM Studio and OpenAI-compatible endpoints are first-class, not a fallback tier. Privacy
First and Local Only routing depend on them working properly, offline, with zero cost accounting.