Align error logging with admin dashboard field conventions

All warn/error log calls now use field names the dashboard's decorate()
function recognizes: `code` for the error code pill, `httpStatus` for the
status badge and severity bucket, `url` for the sidebar link, and
`repoId` for the repository link.

Key changes:
- Streamer errors surface code, httpStatus, url, and nested err in Raw tab
- Nested `{ err: serializeError(e) }` replaced with spread pattern so
  error fields (name, message, status) appear at the top level
- Raw Error objects in catch blocks now go through serializeError()
- Rate limit, token, and PR 404 warnings include code + httpStatus
- Dashboard stack walker traverses both `cause` and `err` chains
- Dashboard Raw tab renders repoId, filePath, upstream*, err, and cause
- trimRawArg recursively trims stacks in nested err/cause chains
- clampPayload strips heavy nested fields before falling back to
  truncated placeholder, preserving flat diagnostic fields

src/core/Gist.ts

@@ -63,7 +63,7 @@ export default class Gist {
       try {
         return this._model.source.accessToken;
       } catch {
-        logger.warn("invalid token", { gistId: this._model.source.gistId });
+        logger.warn("invalid token", { code: "invalid_token", httpStatus: 401, gistId: this._model.source.gistId });
       }
     }
     return config.GITHUB_TOKEN;