feat(settings): Claude category — new-session defaults (T-457)

A Settings → Claude category sets per-user defaults for NEW sessions: model,
effort, and permission mode (generic schema selects persisting
app.claude.default*). The pane reads them at spawn — effort flows through the
existing --effort flag (SpawnSpec.effort); model and permission mode are sent
as control requests right after a fresh (non-resume, non-fork) session starts.
'default'/unset values are no-ops, leaving the CLI's own defaults.

The optional "settings changed — apply to current sessions?" prompt is filed
as T-470: it needs a custom control (a generic select can't prompt), so it's
a clean follow-up on the T-452 escape hatch.

Tests: defaultEffortFlag sentinel handling; the category contributes the three
default fields. The thin pane-side applySessionDefaults is covered by the gate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/builtin/claude/src/extension.dart

@@ -10,7 +10,9 @@ import 'package:clide/builtin/claude/src/claude_session_host.dart';
 import 'package:clide/builtin/claude/src/session_orchestrator.dart';
 import 'package:clide/builtin/claude/src/pane_context_status.dart';
 import 'package:clide/builtin/claude/src/claude_meta_sidebar.dart';
+import 'package:clide/builtin/claude/src/session_defaults.dart';
 import 'package:clide/builtin/claude/src/session_index.dart';
+import 'package:clide/builtin/claude/src/stream_json_session.dart' show kEffortLevels, kFallbackModels, kPermissionModes;
 import 'package:clide/builtin/claude/src/session_storage.dart';
 import 'package:clide/builtin/claude/src/ticket_pick_up.dart';
 import 'package:clide/builtin/claude/src/transcript_reader.dart' show ImageMessage;
@@ -132,6 +134,49 @@ class ClaudeExtension extends ClideExtension {
         ],
       ),
     ),
+    // Claude settings category (T-457) — defaults applied to NEW sessions
+    // (the pane reads these keys at spawn). Effort flows through --effort;
+    // model + permission mode are sent as control requests post-spawn.
+    SettingsCategoryContribution(
+      id: 'claude',
+      category: SettingsCategory(
+        id: 'claude',
+        title: 'Claude',
+        iconName: 'sparkle',
+        priority: 40,
+        sections: [
+          SettingsSection(
+            label: 'New session defaults',
+            fields: [
+              SettingsField(
+                key: kDefaultModelKey,
+                kind: SettingsFieldKind.select,
+                label: 'Model',
+                help: 'Model for new sessions.',
+                defaultValue: 'default',
+                options: [for (final m in kFallbackModels) SettingsOption(value: m.value, label: m.displayName)],
+              ),
+              SettingsField(
+                key: kDefaultEffortKey,
+                kind: SettingsFieldKind.select,
+                label: 'Effort',
+                help: 'Reasoning effort for new sessions (applied via --effort at spawn).',
+                defaultValue: 'high',
+                options: [for (final l in kEffortLevels) SettingsOption(value: l.value, label: l.displayName)],
+              ),
+              SettingsField(
+                key: kDefaultPermissionModeKey,
+                kind: SettingsFieldKind.select,
+                label: 'Permission mode',
+                help: 'Starting permission mode for new sessions.',
+                defaultValue: 'default',
+                options: [for (final p in kPermissionModes) SettingsOption(value: p.value, label: p.displayName)],
+              ),
+            ],
+          ),
+        ],
+      ),
+    ),
     // T-171: agent roster controls (D-6 CLI/UI parity).
     // Usage: clide claude.agent.show <sessionId>
     CommandContribution(