feat: multi-provider model support via majordomo (local Ollama, OpenAI-compatible, etc.)

Replace the hardcoded ollama.Cloud binding with majordomo's provider registry,
so Gadfly can target any backend majordomo supports without code changes.

- cmd/gadfly/model.go: resolveModel() — GADFLY_PROVIDER (default ollama-cloud)
  prefixes bare model ids; GADFLY_MODEL may be a full provider/model spec, alias,
  or failover chain (verbatim). GADFLY_BASE_URL constructs openai/ollama/anthropic/
  google directly at a custom endpoint (OpenAI-compatible + local/remote Ollama).
  GADFLY_API_KEY else the provider's standard env var. + buildSpec unit tests.
- run.sh: provider-aware key gate (local Ollama needs none); maps OLLAMA_CLOUD_API_KEY
  -> OLLAMA_API_KEY; provider/base-url/key inherited by the binary. Gadfly-branded comment.
- entrypoint.sh: GADFLY_MODELS alias for OLLAMA_REVIEW_MODELS; provider passthrough.
- examples + README: Models & providers section. Upfront: only the Ollama paths
  (local + OpenAI-compatible-against-Ollama) are tested; OpenAI/Anthropic/Google
  are wired via majordomo but UNTESTED (no spend).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

examples/adversarial-review.yml

@@ -43,6 +43,27 @@ jobs:
           OLLAMA_CLOUD_API_KEY: ${{ secrets.OLLAMA_CLOUD_API_KEY }}
           OLLAMA_REVIEW_MODELS: ${{ vars.OLLAMA_REVIEW_MODELS }}
           GADFLY_ALLOWED_USERS: ${{ vars.GADFLY_ALLOWED_USERS }}
+          # --- Models & providers (optional; default = Ollama Cloud) ----------
+          # Gadfly is majordomo-powered, so it can target other backends. Set a
+          # provider for bare model ids; point at a different endpoint with a
+          # base URL; supply a key (or the provider's standard env var). Examples:
+          #
+          #   Local Ollama daemon (no key):
+          #     GADFLY_PROVIDER: ollama
+          #     GADFLY_MODELS:   qwen2.5-coder:7b
+          #     # GADFLY_BASE_URL: http://my-ollama-host:11434   # if not localhost
+          #
+          #   OpenAI-compatible endpoint (incl. local Ollama's /v1):
+          #     GADFLY_PROVIDER: openai
+          #     GADFLY_BASE_URL: http://localhost:11434/v1
+          #     GADFLY_MODELS:   qwen2.5-coder:7b
+          #
+          #   OpenAI / Anthropic / Google (supported via majordomo, UNTESTED — see README):
+          #     GADFLY_PROVIDER: openai      # then set OPENAI_API_KEY below
+          #     OPENAI_API_KEY: ${{ secrets.OPENAI_API_KEY }}
+          GADFLY_PROVIDER: ${{ vars.GADFLY_PROVIDER }}
+          GADFLY_BASE_URL: ${{ vars.GADFLY_BASE_URL }}
+          GADFLY_MODELS: ${{ vars.GADFLY_MODELS }}
           EVENT_NAME: ${{ github.event_name }}
           PR: ${{ github.event.pull_request.number || github.event.issue.number || github.event.inputs.pr_number }}
           PR_BRANCH: ${{ github.head_ref }}