feat(chat): cache timer for Claude Code subscription models

Per-step and per-turn timers for the gaps between model requests (the
Claude prompt cache expires after 5 min idle). A step gap runs live under
the executing tools from step-complete until its last tool result (or the
next step's output), then is stamped on that step's metrics line beside
its cache badge. A turn gap runs live on the turn's metrics line from
done/error until the next message is sent, then is stamped there. Counts
m:ss up to 10 min, then '10+ min'; colour tracks the 5-min TTL.

Gated on claude/<model>. Pure reducer in core/metrics/cache-timer.ts;
the turn gap is seeded from an idle conversation's lastActivityAt so it
survives a reload. Stamps are in-memory only.

1 files changed, 21 insertions, 0 deletions

diff --git a/src/features/chat/model-select.test.ts b/src/features/chat/model-select.test.ts
index 57de11d..523f1d8 100644
--- a/src/features/chat/model-select.test.ts
+++ b/src/features/chat/model-select.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, it } from "vitest";
 import {
+  isClaudeSubscriptionModel,
   isNamedAgentSelection,
   isVisionModel,
   joinModelName,
@@ -192,3 +193,23 @@ describe("named-agent provider helpers", () => {
     expect(namedAgentSelectionLabel("Coding")).toBe("Named Agents -> Coding");
   });
 });
+
+describe("isClaudeSubscriptionModel", () => {
+  it("is true for claude/<model> (the Claude Code subscription credential)", () => {
+    expect(isClaudeSubscriptionModel("claude/claude-opus-4-6")).toBe(true);
+    expect(isClaudeSubscriptionModel("claude/claude-sonnet-4-6")).toBe(true);
+  });
+
+  it("is false for Claude models served through other credentials", () => {
+    expect(isClaudeSubscriptionModel("opencode-1/claude-sonnet-4.5")).toBe(false);
+    expect(isClaudeSubscriptionModel("openrouter/anthropic/claude-opus-4")).toBe(false);
+    expect(isClaudeSubscriptionModel("openai/gpt-4o")).toBe(false);
+  });
+
+  it("is false for a bare key, a lookalike key, or undefined", () => {
+    expect(isClaudeSubscriptionModel("claude")).toBe(false);
+    expect(isClaudeSubscriptionModel("claude/")).toBe(false);
+    expect(isClaudeSubscriptionModel("claude-2/claude-opus-4-6")).toBe(false);
+    expect(isClaudeSubscriptionModel(undefined)).toBe(false);
+  });
+});