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/GitHubUtils.ts

@@ -62,6 +62,8 @@ export function octokit(token: string) {
     throttle: {
       onRateLimit: (retryAfter, options, _o, retryCount) => {
         logger.warn("github primary rate limit hit", {
+          code: "github_rate_limit",
+          httpStatus: 429,
           method: options.method,
           url: options.url,
           retryAfter,
@@ -73,6 +75,8 @@ export function octokit(token: string) {
       },
       onSecondaryRateLimit: (retryAfter, options, _o, retryCount) => {
         logger.warn("github secondary rate limit hit", {
+          code: "github_secondary_rate_limit",
+          httpStatus: 429,
           method: options.method,
           url: options.url,
           retryAfter,
@@ -193,8 +197,9 @@ export async function getToken(repository: Repository) {
         }
       }
       logger.warn("token refresh failed; falling back", {
+        code: "token_refresh_failed",
+        httpStatus: res.status,
         username: repository.owner.model.username,
-        status: res.status,
       });
       // fall through to the checkToken path / config.GITHUB_TOKEN
     }