harden IPC: reject -prefixed git refs, cap files.read / git.log (T-104)

Three security fixes the consultant flagged:

* git.checkout, git.push now reject branch/remote arguments starting
  with `-` via a top-level validateGitRef helper. `git push` also
  gets a `--` option terminator; checkout can't use `--` without
  changing semantics (it would be parsed as a pathspec), so the
  validator is the only line of defence there.
* files.read caps responses at 10 MB so a single call can't OOM the
  UI on a multi-gigabyte log.
* git.log caps `count` at 1000; git.diff / git.stage cap paths at
  256. Excess is a userError rather than burning subprocess time.

The bigger typed-schema framework (item 1 in T-104) is split out as
T-120 since it needs design discussion alongside T-99.

Co-Authored-By: Claude <noreply@anthropic.com>

test/daemon/git_commands_test.dart

@@ -294,4 +294,45 @@ void main() {
     final r = await call('git.stage', {'paths': 'new.txt'});
     expect(r.ok, isTrue);
   });
+
+  test('git.checkout rejects a -prefixed branch (argv-injection guard)', () async {
+    final r = await call('git.checkout', {'branch': '--upload-pack=evil'});
+    expect(r.ok, isFalse);
+    expect(r.error?.message, contains('branch'));
+  });
+
+  test('git.push rejects a -prefixed remote', () async {
+    final r = await call('git.push', {'remote': '--upload-pack=evil', 'branch': 'main'});
+    expect(r.ok, isFalse);
+    expect(r.error?.message, contains('remote'));
+  });
+
+  test('git.push rejects a -prefixed branch', () async {
+    final r = await call('git.push', {'remote': 'origin', 'branch': '--exec=evil'});
+    expect(r.ok, isFalse);
+    expect(r.error?.message, contains('branch'));
+  });
+
+  test('git.log over the count cap fails as userError', () async {
+    final r = await call('git.log', {'count': 100000});
+    expect(r.ok, isFalse);
+    expect(r.error?.kind, IpcErrorKind.userError);
+    expect(r.error?.message, contains('exceeds cap'));
+  });
+
+  test('git.diff with too many paths fails as userError', () async {
+    final paths = [for (var i = 0; i < 300; i++) 'file_$i.txt'];
+    final r = await call('git.diff', {'paths': paths});
+    expect(r.ok, isFalse);
+    expect(r.error?.kind, IpcErrorKind.userError);
+    expect(r.error?.message, contains('exceeds cap'));
+  });
+
+  test('git.stage with too many paths fails as userError', () async {
+    final paths = [for (var i = 0; i < 300; i++) 'file_$i.txt'];
+    final r = await call('git.stage', {'paths': paths});
+    expect(r.ok, isFalse);
+    expect(r.error?.kind, IpcErrorKind.userError);
+    expect(r.error?.message, contains('exceeds cap'));
+  });
 }