watch the claude process: drain stderr, surface exit (T-361)

The session observed its child only via stdout. Two failure modes:
with --verbose the CLI chats on stderr, and an undrained 64KB pipe
blocks the child mid-turn with zero diagnostics; and nothing watched
the exit code, so a crashed process just looked thoughtful forever.

ClaudeStreamJsonProcess now drains stderr from construction into a
bounded tail buffer, and StreamJsonSession watches exitCode: on death
it flips busy off, clears any unanswerable pending prompt, and emits
a SessionEnd (exit code + stderr tail) — replayed via session.end for
late binders. The pane reports the exit in its status line and logs
the stderr tail; a deliberate dispose suppresses the watch so /clear
and teardown don't read as crashes. Test fakes extend the process
base instead of implementing it, so its defaults carry.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/builtin/claude/claude_meta_sidebar_test.dart

@@ -20,7 +20,7 @@ import '../../helpers/widget_harness.dart';
 // ---------------------------------------------------------------------------
 // Minimal fake process so orchestrator tests don't need a real `claude` binary.
 // ---------------------------------------------------------------------------
-class _FakeProc implements StreamJsonProcess {
+class _FakeProc extends StreamJsonProcess {
   final _ctl = StreamController<String>.broadcast();
   final List<String> writes = [];
   bool killed = false;

test/builtin/claude/claude_pane_test.dart

@@ -25,7 +25,7 @@ import 'package:flutter_test/flutter_test.dart';
 
 import '../../helpers/kernel_fixture.dart';
 
-class _FakeProc implements StreamJsonProcess {
+class _FakeProc extends StreamJsonProcess {
   final _ctl = StreamController<String>.broadcast();
   final List<String> writes = [];
   bool killed = false;

test/builtin/claude/session_lifecycle_test.dart

@@ -18,7 +18,7 @@ import 'package:test/test.dart';
 // Minimal fake process — same as session_orchestrator_test.dart.
 // ---------------------------------------------------------------------------
 
-class _FakeProc implements StreamJsonProcess {
+class _FakeProc extends StreamJsonProcess {
   final _ctl = StreamController<String>.broadcast();
   final List<String> writes = [];
   bool killed = false;

test/builtin/claude/session_orchestrator_test.dart

@@ -7,7 +7,7 @@ import 'package:clide/builtin/claude/src/stream_json_session.dart';
 import 'package:clide/builtin/claude/src/transcript_reader.dart';
 import 'package:flutter_test/flutter_test.dart';
 
-class _FakeProc implements StreamJsonProcess {
+class _FakeProc extends StreamJsonProcess {
   final _ctl = StreamController<String>.broadcast();
   final List<String> writes = [];
   bool killed = false;

test/builtin/claude/stream_json_session_test.dart

@@ -5,17 +5,25 @@ import 'package:clide/builtin/claude/src/stream_json_session.dart';
 import 'package:clide/builtin/claude/src/transcript_reader.dart';
 import 'package:test/test.dart';
 
-class _FakeProc implements StreamJsonProcess {
+class _FakeProc extends StreamJsonProcess {
   final _ctl = StreamController<String>();
   final List<String> writes = [];
   bool killed = false;
 
+  /// Drives the T-361 exit watch; never completes unless a test exits it.
+  final exit = Completer<int>();
+  final List<String> stderr = [];
+
   @override
   Stream<String> get lines => _ctl.stream;
   @override
   void writeLine(String line) => writes.add(line);
   @override
   Future<void> kill() async => killed = true;
+  @override
+  Future<int> get exitCode => exit.future;
+  @override
+  List<String> get stderrTail => stderr;
 
   void emit(String line) => _ctl.add(line);
 }
@@ -745,4 +753,58 @@ void main() {
       expect((r['error'] as Map)['message'], contains('resources/list'));
     });
   });
+
+  // T-361: nothing watched the process itself — a crashed claude just
+  // looked thoughtful forever.
+  group('process exit (T-361)', () {
+    test('exit emits SessionEnd with code + stderr tail and clears busy', () async {
+      final ends = <SessionEnd>[];
+      session.endedStream.listen(ends.add);
+      session.send('do something');
+      await Future<void>.delayed(Duration.zero);
+      expect(session.busy, isTrue, reason: 'a send marks the turn in flight');
+
+      proc.stderr.addAll(['boom: stack', 'fatal: died']);
+      proc.exit.complete(70);
+      await Future<void>.delayed(Duration.zero);
+
+      expect(session.busy, isFalse, reason: 'a dead process is not thinking');
+      expect(ends, hasLength(1));
+      expect(ends.single.exitCode, 70);
+      expect(ends.single.stderrTail, ['boom: stack', 'fatal: died']);
+      expect(session.end, same(ends.single), reason: 'late binders replay via the getter');
+    });
+
+    test('exit clears a pending prompt — it can never be answered', () async {
+      final pendings = <ToolPrompt?>[];
+      session.pendingPromptStream.listen(pendings.add);
+      proc.emit(canUseTool('p1'));
+      await Future<void>.delayed(Duration.zero);
+      expect(session.pendingPrompt, isNotNull);
+
+      proc.exit.complete(1);
+      await Future<void>.delayed(Duration.zero);
+      expect(session.pendingPrompt, isNull);
+      expect(pendings.last, isNull, reason: 'the composer swaps back from the prompt UI');
+    });
+
+    test('a deliberate dispose suppresses the exit watch', () async {
+      final p = _FakeProc();
+      final s = StreamJsonSession(p)..start();
+      await s.dispose();
+      p.exit.complete(9); // the kill's exit must not surface as a crash
+      await Future<void>.delayed(Duration.zero);
+      expect(s.end, isNull);
+    });
+  });
+
+  group('BoundedLineBuffer', () {
+    test('keeps only the last cap lines', () {
+      final b = BoundedLineBuffer(cap: 3);
+      for (var i = 0; i < 5; i++) {
+        b.add('line $i');
+      }
+      expect(b.lines, ['line 2', 'line 3', 'line 4']);
+    });
+  });
 }

test/builtin/claude/ticket_pick_up_test.dart

@@ -14,7 +14,7 @@ import 'package:flutter_test/flutter_test.dart';
 
 import '../../helpers/fake_ipc.dart';
 
-class _FakeProc implements StreamJsonProcess {
+class _FakeProc extends StreamJsonProcess {
   final _ctl = StreamController<String>.broadcast();
   final List<String> writes = [];
   @override