fix(claude): show the Fast toggle on Opus 5 (#2939)

The Claude model manifest declares fast-mode support per model, and the
`claude-opus-5` entry was added without `supportsFastMode`. Claude Code
supports fast mode on Opus 5, so the composer and Command Center hid a
toggle the model actually has.

Set the flag. Everything downstream already keys off it:
`claudeManifestModelSupportsFastMode` gates `buildClaudeFeatures`, which
feeds `listFeatures`, the session `features` getter, `setFeature`, and
the `setModel` auto-clear.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

packages/server/src/server/agent/providers/claude/agent.test.ts

@@ -679,6 +679,14 @@ describe("ClaudeAgentSession features", () => {
       }),
     ).resolves.toEqual([expect.objectContaining({ id: "fast_mode", value: false })]);
 
+    await expect(
+      client.listFeatures({
+        provider: "claude",
+        cwd: process.cwd(),
+        model: "claude-opus-5",
+      }),
+    ).resolves.toEqual([expect.objectContaining({ id: "fast_mode", value: false })]);
+
     await expect(
       client.listFeatures({
         provider: "claude",
@@ -687,6 +695,14 @@ describe("ClaudeAgentSession features", () => {
       }),
     ).resolves.toEqual([]);
 
+    await expect(
+      client.listFeatures({
+        provider: "claude",
+        cwd: process.cwd(),
+        model: "claude-sonnet-5",
+      }),
+    ).resolves.toEqual([]);
+
     await expect(
       client.listFeatures({
         provider: "claude",

packages/server/src/server/agent/providers/claude/model-manifest.ts

@@ -43,6 +43,7 @@ export const CLAUDE_MODEL_MANIFEST = [
     contextWindowMaxTokens: 1_000_000,
     effortLevels: CLAUDE_EFFORT_LEVELS.xhigh,
     supportsThinkingDisabled: true,
+    supportsFastMode: true,
   },
   {
     id: "claude-fable-5",

packages/server/src/server/agent/providers/claude/models.test.ts

@@ -497,4 +497,10 @@ describe("claudeManifestModelSupportsFastMode", () => {
     expect(claudeManifestModelSupportsFastMode("openrouter/anthropic/claude-opus-4-8")).toBe(false);
     expect(claudeManifestModelSupportsFastMode("claude-opus-4-8-20260101")).toBe(true);
   });
+
+  it("supports fast mode on Opus 5 but not on other Claude 5 models", () => {
+    expect(claudeManifestModelSupportsFastMode("claude-opus-5")).toBe(true);
+    expect(claudeManifestModelSupportsFastMode("claude-sonnet-5")).toBe(false);
+    expect(claudeManifestModelSupportsFastMode("claude-fable-5")).toBe(false);
+  });
 });