fix(agent): correctly strip <final> tags from reasoning providers

- Added src/utils/provider-utils.ts to track reasoning provider logic
- Updated isReasoningTagProvider to loosely match 'google-antigravity' (fixes sub-models)
- Enabled enforceFinalTag in reply.ts when provider matches
- Verified <final> tag stripping logic in pi-embedded-subscribe.ts
- Updated pi-embedded-runner to use consistent provider check for prompt hints

src/utils/provider-utils.ts

@@ -0,0 +1,29 @@
+/**
+ * Utility functions for provider-specific logic and capabilities.
+ */
+
+/**
+ * Returns true if the provider requires reasoning to be wrapped in tags
+ * (e.g. <think> and <final>) in the text stream, rather than using native
+ * API fields for reasoning/thinking.
+ */
+export function isReasoningTagProvider(provider: string | undefined | null): boolean {
+  if (!provider) return false;
+  const normalized = provider.trim().toLowerCase();
+  
+  // Check for exact matches or known prefixes/substrings for reasoning providers
+  if (
+    normalized === "ollama" ||
+    normalized === "google-gemini-cli" ||
+    normalized === "google-generative-ai"
+  ) {
+    return true;
+  }
+  
+  // Handle google-antigravity and its model variations (e.g. google-antigravity/gemini-3)
+  if (normalized.includes("google-antigravity")) {
+    return true;
+  }
+  
+  return false;
+}