feat(adapters/gemini-local): add auth detection, turn-limit handling, sandbox, and approval modes

Incorporate improvements from PR #13 and #105 into the gemini-local adapter:

- Add detectGeminiAuthRequired() for runtime auth failure detection with
  errorCode: "gemini_auth_required" on execution results
- Add isGeminiTurnLimitResult() to detect exit code 53 / turn_limit status
  and clear session to prevent stuck sessions on next heartbeat
- Add describeGeminiFailure() for structured error messages from parsed
  result events including errors array extraction
- Return parsed resultEvent in resultJson instead of raw stdout/stderr
- Add isRetry guard to prevent stale session ID fallback after retry
- Replace boolean yolo with approvalMode string (default/auto_edit/yolo)
  with backwards-compatible config.yolo fallback
- Add sandbox config option (--sandbox / --sandbox=none)
- Add GOOGLE_GENAI_USE_GCA auth detection in environment test
- Consolidate auth detection regex into shared detectGeminiAuthRequired()
- Add gemini-2.0-flash and gemini-2.0-flash-lite model IDs

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/adapters/gemini-local/src/server/index.ts

@@ -1,6 +1,12 @@
 export { execute } from "./execute.js";
 export { testEnvironment } from "./test.js";
-export { parseGeminiJsonl, isGeminiUnknownSessionError } from "./parse.js";
+export {
+  parseGeminiJsonl,
+  isGeminiUnknownSessionError,
+  describeGeminiFailure,
+  detectGeminiAuthRequired,
+  isGeminiTurnLimitResult,
+} from "./parse.js";
 import type { AdapterSessionCodec } from "@paperclipai/adapter-utils";
 
 function readNonEmptyString(value: unknown): string | null {