Remove the Anthropic path; the gateway speaks one wire protocol

The platform now runs on Groq by default, through the OpenAI-compatible
chat-completions shape. That shape is not one vendor — Gemini, OpenRouter,
Together, vLLM and a local Ollama serve it too — so moving again stays
configuration rather than code.

Two things in the deleted file were not Anthropic's and would have gone
with it silently:

  withRetry / MaxAttempts / retryBackoff were defined in anthropic.go and
  CALLED BY openai.go. Deleting the file wholesale would have removed the
  retry policy of the provider that survived, and nothing in openai.go
  mentions it, so the loss would have been invisible until the next 429.
  The policy is a property of this platform's runs, not of a vendor's API;
  it now lives in retry.go where no provider can carry it off.

  StreamComplete had the same problem and moves to gateway.go, beside the
  Streamer interface whose comment already referenced it.

Three stale-configuration failures are now refused at startup instead of
being ignored. Each was verified firing through the real config.Load():

  MODEL_PROVIDER=anthropic — named separately from every other wrong value
  because it used to be correct. Ignoring it gives a stack that believes it
  is on Claude while every run goes to Groq and is billed there.

  ANTHROPIC_API_KEY set while MODEL_API_KEY is empty. Ignoring a key an
  operator did set is the worst version of this: they fail every run on a
  missing credential they are looking straight at.

  A leftover claude-* model id, naming the tier that carries it. This is
  the check the previous commit's error-detail work was diagnosing: such an
  id is accepted by this process, rejected by the provider, and 400s on
  EVERY run. "A model is wrong" does not say which of three lines to edit.

Defaults ship as a matched pair. defaultBaseURL and the three tier ids are
one decision, not four: an id is only meaningful against the service that
serves it, and a Groq id on an OpenAI base URL is the same failure from the
other side. The tiers also stop being one model — a tier whose cost does
not differ is a distinction that buys nothing.

Verified end to end against a stub of the wire, driving the real wiring
(config.Load in production mode, gateway.New, StreamComplete): streamed
deltas, tool-call decoding, the loopback credential exemption, and usage
totalling 150 rather than 190 — the cached-prefix subtraction still holds.

gofmt clean, go vet clean, 14/14 non-DB packages pass. httpserver still
needs a reachable database.

NOT verified: the I7 planted-injection eval. Removing this path removed the
only model whose refusal behaviour had been measured against it, so the new
default is unproven there until `make eval-live` runs with a real key. The
Groq model ids should also be confirmed against Groq's current lineup.
Flagged in CLAUDE.md §12 and docs/handover.md.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PJvibeSc1JYXjatankqM1g

infrastructure/docker-compose.yml

@@ -92,8 +92,13 @@ services:
       # without it answers 404 on /agents/{id}/runs and reports three fewer
       # endpoints on /version. Empty by default: absent is a working API
       # without Owliver, which is a legitimate way to run this.
-      # Provider selection. Empty MODEL_PROVIDER means anthropic, so a stack
-      # that predates the second provider comes up exactly as it did.
+      # Provider selection. Empty MODEL_PROVIDER means openai — the only
+      # implementation — and empty MODEL_BASE_URL means Groq.
+      #
+      # ANTHROPIC_API_KEY is passed through DELIBERATELY even though nothing
+      # reads it: a host that still exports it gets a loud startup failure
+      # telling it to rename the variable, instead of a container that silently
+      # boots with no credential and fails every agent run.
       MODEL_PROVIDER: ${MODEL_PROVIDER:-}
       MODEL_BASE_URL: ${MODEL_BASE_URL:-}
       MODEL_API_KEY: ${MODEL_API_KEY:-}