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>

entrypoint.sh

@@ -31,7 +31,11 @@
 #   COMMENT_ID            comment id, for the 👀 reaction (issue_comment only)
 #   ACTOR                 github.actor (the user who triggered)
 # Optional config:
-#   OLLAMA_REVIEW_MODELS  comma-separated model ids (default below)
+#   GADFLY_MODELS         comma-separated model ids/specs (alias: OLLAMA_REVIEW_MODELS)
+#   GADFLY_PROVIDER       majordomo provider for bare model ids (default ollama-cloud;
+#                         e.g. "ollama" local, "openai", "anthropic", "google")
+#   GADFLY_BASE_URL       override backend endpoint (OpenAI/Ollama-compatible servers)
+#   GADFLY_API_KEY        provider key (else provider's standard env: OPENAI_API_KEY, …)
 #   GADFLY_TRIGGER_PHRASE comment phrase that triggers a re-review (default "@gadfly review")
 #   GADFLY_ALLOWED_USERS  comma-separated usernames allowed to comment-trigger;
 #                         empty => fall back to "is a repo collaborator"
@@ -119,8 +123,11 @@ git clone --depth=1 --branch "$BRANCH" "$CLONE_URL" "$REPO_DIR" 2>/dev/null \
   || die "clone of ${REPO_PATH}@${BRANCH} failed"
 
 # --- review once per model -------------------------------------------------
-MODELS="${OLLAMA_REVIEW_MODELS:-$DEFAULT_MODELS}"
-log "models: ${MODELS}"
+# GADFLY_MODELS is the provider-agnostic name; OLLAMA_REVIEW_MODELS is kept as a
+# back-compat alias. GADFLY_PROVIDER / GADFLY_BASE_URL / GADFLY_API_KEY and any
+# provider key envs (OPENAI_API_KEY, …) are inherited by run.sh and the binary.
+MODELS="${GADFLY_MODELS:-${OLLAMA_REVIEW_MODELS:-$DEFAULT_MODELS}}"
+log "provider: ${GADFLY_PROVIDER:-ollama-cloud}; models: ${MODELS}"
 IFS=',' read -ra ARR <<< "$MODELS" || true
 for raw in "${ARR[@]}"; do
   m="$(echo "$raw" | tr -d '[:space:]')"