FN-6132: remove unused base SHA from branch authority check

Fix the branch authority helper and caller so the squash merge stays lint-clean.

- drop the unused `baseSha` parameter from `isBranchAuthoritativeForTask`
- update the merger authority probe to call the helper with the remaining arguments

Files changed:
 packages/engine/src/branch-conflicts.ts | 1 -
 packages/engine/src/merger.ts           | 2 +-
 2 files changed, 1 insertion(+), 2 deletions(-)

Fusion-Task-Id: FN-6132
Fusion-Task-Lineage: c8d8352d-67f7-498b-8b6d-254375d42417

packages/engine/src/branch-conflicts.ts

@@ -377,7 +377,6 @@ export async function isBranchAuthoritativeForTask(
   repoDir: string,
   branch: string,
   taskId: string,
-  baseSha?: string,
 ): Promise<{ ok: true } | { ok: false; reason: string }> {
   try {
     await revParse(repoDir, `refs/heads/${branch}`);

packages/engine/src/merger.ts

@@ -4668,7 +4668,7 @@ export async function commitOrAmendMergeWithFixes(
     // that this attempt's integration worktree never advanced. Reset rootDir
     // to preAttemptHeadSha so the next merge attempt starts clean, and tag
     // the result so the caller's diagnostic includes that context.
-    const authority = await isBranchAuthoritativeForTask(rootDir, branch, taskId, preAttemptHeadSha).catch(
+    const authority = await isBranchAuthoritativeForTask(rootDir, branch, taskId).catch(
       () => ({ ok: false as const, reason: "authority-probe-failed" }),
     );
     if (authority.ok) {