fix(repos create): default new repos to main, not master

`git init` without `-b` inherits the ambient `init.defaultBranch` config,
which is `master` unless the host has configured otherwise, so `pkgmgr
repos create` produced master-based repositories. On top of that, the
push fallback renamed an already-correct `main` branch back to `master`
whenever the initial push failed (missing remote, auth error).

`init()` now requires an explicit `initial_branch` and runs
`git init -b <branch>`; the bootstrapper passes `main` and no longer
falls back to `master` on push failure.

`resolve_base_branch()` keeps its main -> master fallback, since it reads
existing repositories that may legitimately use master.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/unit/pkgmgr/actions/repository/create/test_git_bootstrap_default_branch.py

@@ -0,0 +1,60 @@
+from __future__ import annotations
+
+import unittest
+from unittest.mock import patch
+
+from pkgmgr.actions.repository.create.git_bootstrap import GitBootstrapper
+from pkgmgr.core.git.commands import GitPushUpstreamError
+
+
+class TestGitBootstrapperDefaultBranch(unittest.TestCase):
+    def test_init_repo_uses_main_as_initial_branch(self):
+        with (
+            patch("pkgmgr.actions.repository.create.git_bootstrap.init") as init,
+            patch("pkgmgr.actions.repository.create.git_bootstrap.add_all"),
+            patch("pkgmgr.actions.repository.create.git_bootstrap.commit"),
+        ):
+            GitBootstrapper().init_repo("/repo", preview=False)
+
+        init.assert_called_once_with(
+            initial_branch="main", cwd="/repo", preview=False
+        )
+
+    def test_push_default_branch_pushes_main(self):
+        with (
+            patch(
+                "pkgmgr.actions.repository.create.git_bootstrap.branch_move"
+            ) as branch_move,
+            patch(
+                "pkgmgr.actions.repository.create.git_bootstrap.push_upstream"
+            ) as push_upstream,
+        ):
+            GitBootstrapper().push_default_branch("/repo", preview=False)
+
+        branch_move.assert_called_once_with("main", cwd="/repo", preview=False)
+        push_upstream.assert_called_once_with(
+            "origin", "main", cwd="/repo", preview=False
+        )
+
+    def test_failed_push_does_not_fall_back_to_master(self):
+        with (
+            patch(
+                "pkgmgr.actions.repository.create.git_bootstrap.branch_move"
+            ) as branch_move,
+            patch(
+                "pkgmgr.actions.repository.create.git_bootstrap.push_upstream",
+                side_effect=GitPushUpstreamError("boom"),
+            ) as push_upstream,
+        ):
+            GitBootstrapper().push_default_branch("/repo", preview=False)
+
+        self.assertEqual(
+            [call.args[0] for call in branch_move.call_args_list], ["main"]
+        )
+        self.assertEqual(
+            [call.args[1] for call in push_upstream.call_args_list], ["main"]
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()