fix: Add structured output fallback for non-Claude models in app spec generation

This fixes the app spec generation failing for non-Claude models (Cursor, Gemini,
OpenCode, Copilot) that don't support structured output capabilities.

Changes:
- Add `supportsStructuredOutput()` utility function in @automaker/types to
  centralize model capability detection
- Update generate-features-from-spec.ts:
  - Add explicit JSON instructions for non-Claude/Codex models
  - Define featuresOutputSchema for structured output
  - Pre-extract JSON from text responses using extractJsonWithArray
  - Handle both structured_output and text responses properly
- Update generate-spec.ts:
  - Replace isCursorModel with supportsStructuredOutput for consistency
- Update sync-spec.ts:
  - Add techStackOutputSchema for structured output
  - Add JSON extraction fallback for text responses
  - Handle both structured_output and text parsing
- Update validate-issue.ts:
  - Use supportsStructuredOutput for cleaner capability detection

The fix follows the same pattern used in generate-spec.ts where non-Claude models
receive explicit JSON formatting instructions in the prompt and responses are
parsed using extractJson utilities.

Fixes #669

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

libs/types/src/index.ts

@@ -272,6 +272,7 @@ export {
   getBareModelId,
   normalizeModelString,
   validateBareModelId,
+  supportsStructuredOutput,
 } from './provider-utils.js';
 
 // Model migration utilities

libs/types/src/provider-utils.ts

@@ -345,6 +345,34 @@ export function normalizeModelString(model: string | undefined | null): string {
   return model;
 }
 
+/**
+ * Check if a model supports structured output (JSON schema)
+ *
+ * Structured output is a feature that allows the model to return responses
+ * conforming to a JSON schema. Currently supported by:
+ * - Claude models (native Anthropic API support)
+ * - Codex/OpenAI models (via response_format with json_schema)
+ *
+ * Models that do NOT support structured output:
+ * - Cursor models (uses different API format)
+ * - OpenCode models (various backend providers)
+ * - Gemini models (different API)
+ * - Copilot models (proxy to various backends)
+ *
+ * @param model - Model string to check
+ * @returns true if the model supports structured output
+ *
+ * @example
+ * supportsStructuredOutput('sonnet') // true (Claude)
+ * supportsStructuredOutput('claude-sonnet-4-20250514') // true (Claude)
+ * supportsStructuredOutput('codex-gpt-5.2') // true (Codex/OpenAI)
+ * supportsStructuredOutput('cursor-auto') // false
+ * supportsStructuredOutput('gemini-2.5-pro') // false
+ */
+export function supportsStructuredOutput(model: string | undefined | null): boolean {
+  return isClaudeModel(model) || isCodexModel(model);
+}
+
 /**
  * Validate that a model ID does not contain a provider prefix
  *