fix: fail closed on unknown audit default branch

src/bin/choir/uds_server.mbt

@@ -3604,6 +3604,10 @@ async test "heartbeat tick loop triggers automerge sweep path via process_poller
   }
   let capture = async fn(cmd : @workspace.Command) -> (Int, String) {
     @async.sleep(0)
+    if cmd.program == "git" &&
+      cmd.args == ["symbolic-ref", "--short", "refs/remotes/origin/HEAD"] {
+      return (0, "origin/main\n")
+    }
     let repo_view_json =
       #|{"owner":{"login":"brickfrog"},"name":"choir"}
     if (

src/server/handler_automerge.mbt

@@ -396,10 +396,14 @@ pub async fn maybe_trigger_automerge_for_pending_event(
       )
     None => []
   }
-  let default_branch = @tools.detect_default_branch_with_capture(
+  let default_branch_result = @tools.detect_default_branch_result_with_capture(
     state.config.project_dir,
     capture,
   )
+  let default_branch = match default_branch_result {
+    Ok(branch) => branch
+    Err(_) => "main"
+  }
   let audit_receipt_state = @tools.audit_receipt_advisory_state_for_tracked_pr(
     state.config.project_dir,
     tracked.parent_branch,
@@ -409,6 +413,7 @@ pub async fn maybe_trigger_automerge_for_pending_event(
     capture,
     read_file,
     path_exists,
+    default_branch_result=Some(default_branch_result),
   )
   let decision = @poller.merge_policy_decision_for_tracked_pr(
     tracked,

src/server/handler_poll_delivery.mbt

@@ -2329,10 +2329,14 @@ async fn ServerState::refresh_tracked_pr_audit_receipt_states(
   if tracked_prs.length() == 0 {
     return
   }
-  let default_branch = @tools.detect_default_branch_with_capture(
+  let default_branch_result = @tools.detect_default_branch_result_with_capture(
     self.config.project_dir,
     capture,
   )
+  let default_branch = match default_branch_result {
+    Ok(branch) => branch
+    Err(_) => "main"
+  }
   self.cached_default_branch = default_branch
   for tracked in tracked_prs {
     match self.poller.get(tracked.pr_number) {
@@ -2346,6 +2350,7 @@ async fn ServerState::refresh_tracked_pr_audit_receipt_states(
           capture,
           read_file,
           path_exists,
+          default_branch_result=Some(default_branch_result),
         )
         self.poller.restore({ ..current, audit_receipt_state, })
       }
@@ -2430,10 +2435,20 @@ async fn deliver_pending_poll_events(
   if retried_events.length() > 0 {
     state.request_goal_judge_tick()
   }
-  let default_branch = if pending_events.length() > 0 {
-    @tools.detect_default_branch_with_capture(state.config.project_dir, capture)
+  let default_branch_result = if pending_events.length() > 0 {
+    Some(
+      @tools.detect_default_branch_result_with_capture(
+        state.config.project_dir,
+        capture,
+      ),
+    )
   } else {
-    ""
+    None
+  }
+  let default_branch = match default_branch_result {
+    Some(Ok(branch)) => branch
+    Some(Err(_)) => "main"
+    None => ""
   }
   for i = 0; i < pending_events.length(); i = i + 1 {
     let event = pending_events[i]
@@ -2507,6 +2522,7 @@ async fn deliver_pending_poll_events(
             capture,
             audit_receipt_read,
             audit_receipt_exists,
+            default_branch_result~,
           )
         }
       None => @types.AuditReceiptState::NotRequired
@@ -2847,10 +2863,20 @@ async fn ServerState::sweep_tracked_prs_for_automerge(
   for tracked in self.poller.tracked.values() {
     tracked_prs.push(tracked)
   }
-  let default_branch = if tracked_prs.length() > 0 {
-    @tools.detect_default_branch_with_capture(self.config.project_dir, capture)
+  let default_branch_result = if tracked_prs.length() > 0 {
+    Some(
+      @tools.detect_default_branch_result_with_capture(
+        self.config.project_dir,
+        capture,
+      ),
+    )
   } else {
-    ""
+    None
+  }
+  let default_branch = match default_branch_result {
+    Some(Ok(branch)) => branch
+    Some(Err(_)) => "main"
+    None => ""
   }
   for tracked in tracked_prs {
     let current = match self.poller.get(tracked.pr_number) {
@@ -2879,6 +2905,7 @@ async fn ServerState::sweep_tracked_prs_for_automerge(
       capture,
       read_file,
       path_exists,
+      default_branch_result~,
     )
     let gate_ready = @poller.tl_tracked_pr_merge_gate_snapshot_ready(
       current,