fix(cli): fall back to main when current branch has no remote counterpart

`hermes update` crashed with CalledProcessError when run on a local-only
branch (e.g. fix/stoicneko) because `git rev-list HEAD..origin/{branch}`
fails when origin/{branch} doesn't exist. Now verifies the remote branch
exists first and falls back to origin/main.

hermes_cli/main.py

@@ -2056,7 +2056,15 @@ def cmd_update(args):
             check=True
         )
         branch = result.stdout.strip()
-        
+
+        # Fall back to main if the current branch doesn't exist on the remote
+        verify = subprocess.run(
+            git_cmd + ["rev-parse", "--verify", f"origin/{branch}"],
+            cwd=PROJECT_ROOT, capture_output=True, text=True,
+        )
+        if verify.returncode != 0:
+            branch = "main"
+
         # Check if there are updates
         result = subprocess.run(
             git_cmd + ["rev-list", f"HEAD..origin/{branch}", "--count"],

tests/hermes_cli/test_cmd_update.py

@@ -0,0 +1,107 @@
+"""Tests for cmd_update — branch fallback when remote branch doesn't exist."""
+
+import subprocess
+from types import SimpleNamespace
+from unittest.mock import patch
+
+import pytest
+
+from hermes_cli.main import cmd_update, PROJECT_ROOT
+
+
+def _make_run_side_effect(branch="main", verify_ok=True, commit_count="0"):
+    """Build a side_effect function for subprocess.run that simulates git commands."""
+
+    def side_effect(cmd, **kwargs):
+        joined = " ".join(str(c) for c in cmd)
+
+        # git rev-parse --abbrev-ref HEAD  (get current branch)
+        if "rev-parse" in joined and "--abbrev-ref" in joined:
+            return subprocess.CompletedProcess(cmd, 0, stdout=f"{branch}\n", stderr="")
+
+        # git rev-parse --verify origin/{branch}  (check remote branch exists)
+        if "rev-parse" in joined and "--verify" in joined:
+            rc = 0 if verify_ok else 128
+            return subprocess.CompletedProcess(cmd, rc, stdout="", stderr="")
+
+        # git rev-list HEAD..origin/{branch} --count
+        if "rev-list" in joined:
+            return subprocess.CompletedProcess(cmd, 0, stdout=f"{commit_count}\n", stderr="")
+
+        # Fallback: return a successful CompletedProcess with empty stdout
+        return subprocess.CompletedProcess(cmd, 0, stdout="", stderr="")
+
+    return side_effect
+
+
+@pytest.fixture
+def mock_args():
+    return SimpleNamespace()
+
+
+class TestCmdUpdateBranchFallback:
+    """cmd_update falls back to main when current branch has no remote counterpart."""
+
+    @patch("shutil.which", return_value=None)
+    @patch("subprocess.run")
+    def test_update_falls_back_to_main_when_branch_not_on_remote(
+        self, mock_run, _mock_which, mock_args, capsys
+    ):
+        mock_run.side_effect = _make_run_side_effect(
+            branch="fix/stoicneko", verify_ok=False, commit_count="3"
+        )
+
+        cmd_update(mock_args)
+
+        commands = [" ".join(str(a) for a in c.args[0]) for c in mock_run.call_args_list]
+
+        # rev-list should use origin/main, not origin/fix/stoicneko
+        rev_list_cmds = [c for c in commands if "rev-list" in c]
+        assert len(rev_list_cmds) == 1
+        assert "origin/main" in rev_list_cmds[0]
+        assert "origin/fix/stoicneko" not in rev_list_cmds[0]
+
+        # pull should use main, not fix/stoicneko
+        pull_cmds = [c for c in commands if "pull" in c]
+        assert len(pull_cmds) == 1
+        assert "main" in pull_cmds[0]
+
+    @patch("shutil.which", return_value=None)
+    @patch("subprocess.run")
+    def test_update_uses_current_branch_when_on_remote(
+        self, mock_run, _mock_which, mock_args, capsys
+    ):
+        mock_run.side_effect = _make_run_side_effect(
+            branch="main", verify_ok=True, commit_count="2"
+        )
+
+        cmd_update(mock_args)
+
+        commands = [" ".join(str(a) for a in c.args[0]) for c in mock_run.call_args_list]
+
+        rev_list_cmds = [c for c in commands if "rev-list" in c]
+        assert len(rev_list_cmds) == 1
+        assert "origin/main" in rev_list_cmds[0]
+
+        pull_cmds = [c for c in commands if "pull" in c]
+        assert len(pull_cmds) == 1
+        assert "main" in pull_cmds[0]
+
+    @patch("shutil.which", return_value=None)
+    @patch("subprocess.run")
+    def test_update_already_up_to_date(
+        self, mock_run, _mock_which, mock_args, capsys
+    ):
+        mock_run.side_effect = _make_run_side_effect(
+            branch="main", verify_ok=True, commit_count="0"
+        )
+
+        cmd_update(mock_args)
+
+        captured = capsys.readouterr()
+        assert "Already up to date!" in captured.out
+
+        # Should NOT have called pull
+        commands = [" ".join(str(a) for a in c.args[0]) for c in mock_run.call_args_list]
+        pull_cmds = [c for c in commands if "pull" in c]
+        assert len(pull_cmds) == 0