fix(github): scope PR selection and clear rejected cached links

Apply the default-branch policy while probing each repository, so an
integration PR cannot preempt a valid result or suppress the same number
in another repository. Check branch heads by repository and server.

Invalidate remote defaults when their review scope or SSH provider changes.
Report rejected PR URLs through an optional refresh-result field, so merged
integration PRs clear from both caches even at the current worktree head.
Share the manual and coordinator cache update implementation.

Co-authored-by: petersindex <287557875+petersindex@users.noreply.github.com>

src/shared/github/pull-request-for-branch-outcome.test.ts

@@ -19,6 +19,13 @@ describe('normalizeGitHubPRForBranchOutcome', () => {
     expect(normalizeGitHubPRForBranchOutcome(outcome, 20)).toBe(outcome)
   })
 
+  it.each([
+    { kind: 'no-pr' as const, fetchedAt: 10 },
+    { kind: 'no-pr' as const, fetchedAt: 10, rejectedPRUrls: [PR.url] }
+  ])('preserves no-PR responses from older and newer hosts', (outcome) => {
+    expect(normalizeGitHubPRForBranchOutcome(outcome, 20)).toEqual(outcome)
+  })
+
   it('normalizes legacy PRInfo and null responses', () => {
     expect(normalizeGitHubPRForBranchOutcome(PR, 20)).toEqual({
       kind: 'found',

src/shared/github/pull-request-refresh-types.ts

@@ -22,7 +22,7 @@ export type PRRefreshUpstreamErrorType = PRRefreshErrorType
 
 export type PRRefreshOutcome =
   | { kind: 'found'; pr: PRInfo; fetchedAt: number }
-  | { kind: 'no-pr'; fetchedAt: number }
+  | { kind: 'no-pr'; fetchedAt: number; rejectedPRUrls?: string[] }
   | {
       kind: 'upstream-error'
       errorType: PRRefreshErrorType