Serve the advisor default branch from config instead of a repos.get call (#8582)

Stack from [ghstack](https://github.com/ezyang/ghstack/tree/0.14.0)
(oldest at bottom):
* #8584
* __->__ #8582
* #8580
* #8579
* #8578
* #8576

**Impact:** AI CI Advisor auto-dispatch path (Dr.CI cron)
**Risk:** low

## What
Adds a `defaultBranch` field to `AdvisorRepoConfig` and reads it in
`dispatchAdvisorWorkflow` instead of looking the default branch up via
the GitHub `repos.get` REST call on every dispatch.

## Why
The PyTorchBot GitHub App installation intermittently hits its shared
hourly REST rate limit, causing random 403s. This drops one
installation-token call per advisor dispatch (~12k/mo) by moving a value
that never changes into the per-repo config, which is already the single
source of truth for advisor settings.

Signed-off-by: Jean Schmidt <contato@jschmidt.me>

Author: jeanschmidt

torchci/lib/advisor/advisorConfig.ts

@@ -36,6 +36,9 @@ export interface AdvisorRepoConfig {
   // The workflow_dispatch file (on the repo's default branch) that runs the
   // advisor analysis for this repo.
   workflowFile: string;
+  // The repo's default branch (e.g. "main") the advisor dispatches against. Kept
+  // as config so the dispatch path needs no repos.get REST call per run.
+  defaultBranch: string;
   // Auto-dispatch bails entirely if a PR has more than this many NEW failures
   // (outage guard), unless the PR carries an OUTAGE_GUARD_BYPASS_LABELS label.
   // Falls back to DEFAULT_MAX_NEW_FAILURES when unset.
@@ -54,6 +57,7 @@ export interface AdvisorRepoConfig {
 export const ADVISOR_REPOS: Record<string, AdvisorRepoConfig> = {
   "pytorch/pytorch": {
     workflowFile: "claude-autorevert-advisor.yml",
+    defaultBranch: "main",
     maxNewFailures: 8,
     maxDispatchPerPr: 32,
   },

torchci/lib/advisor/advisorDispatch.ts

@@ -187,9 +187,7 @@ export async function dispatchAdvisorWorkflow(
   const botOctokit = await getOctokit(owner, repo);
   const jobPattern = jobNameToBasePattern(jobName);
 
-  // Look up default branch (usually "main") instead of hardcoding
-  const repoData = await botOctokit.rest.repos.get({ owner, repo });
-  const defaultBranch = repoData.data.default_branch;
+  const defaultBranch = cfg.defaultBranch;
 
   // Fetch merge base SHA from GitHub
   let resolvedMergeBase = params.mergeBaseSha || "";