Method resolveBundleTrunkRef

src/node/runtime/SSHRuntime.ts:1528–1557

* Resolve the bundle staging ref for the trunk branch. * Returns refs/mux-bundle/ if it exists, otherwise falls back * to the first available ref under refs/mux-bundle/ (handles main vs master * mismatches). Returns null if no bundle refs exist.

(
    baseRepoPathArg: string,
    trunkBranch: string,
    abortSignal?: AbortSignal
  )

   * mismatches). Returns null if no bundle refs exist.
   */
  private async resolveBundleTrunkRef(
    baseRepoPathArg: string,
    trunkBranch: string,
    abortSignal?: AbortSignal
  ): Promise<string | null> {
    // Preferred: exact match for the expected trunk branch.
    const preferredRef = `${BUNDLE_REF_PREFIX}${trunkBranch}`;
    const check = await execBuffered(
      this,
      `git -C ${baseRepoPathArg} rev-parse --verify ${shescape.quote(preferredRef)}`,
      { cwd: "/tmp", timeout: 10, abortSignal }
    );
    if (check.exitCode === 0) {
      return preferredRef;
    }

    // Fallback: pick the first ref under refs/mux-bundle/ (handles main↔master mismatch).
    const listResult = await execBuffered(
      this,
      `git -C ${baseRepoPathArg} for-each-ref --format='%(refname)' ${BUNDLE_REF_PREFIX} --count=1`,
      { cwd: "/tmp", timeout: 10, abortSignal }
    );
    const fallbackRef = listResult.stdout.trim();
    if (listResult.exitCode === 0 && fallbackRef.length > 0) {
      log.info(`Bundle trunk ref mismatch: expected ${preferredRef}, using ${fallbackRef}`);
      return fallbackRef;
    }

    return null;
  }

  private async remoteTrackingBranchExists(
    baseRepoPathArg: string,