feat(coding-agent): add configurable title system prompt for sessions

- add discovery of `TITLE_SYSTEM.md` and pass it through interactive startup context
- route custom title prompts to online and local tiny title generators via protocol
- update session-title docs and changelog with override behavior
- add tests for prompt discovery, forwarding, and fallback to bundled title prompts

packages/coding-agent/test/title-generator.test.ts

@@ -71,6 +71,49 @@ describe("title generator", () => {
 		});
 	});
 
+	it("uses the bundled default prompt when no title prompt file is resolved", async () => {
+		const model = getModelOrThrow("claude-sonnet-4-5");
+		const completeSimpleMock = vi.spyOn(ai, "completeSimple").mockResolvedValue({
+			stopReason: "stop",
+			content: [{ type: "toolCall", id: "call-title", name: "set_title", arguments: { title: "Default Prompt" } }],
+		} as never);
+
+		await generateSessionTitle("Investigate the resolver", createRegistry(model), createSettings(model));
+
+		const request = completeSimpleMock.mock.calls[0]?.[1] as { systemPrompt?: string[] } | undefined;
+		expect(request?.systemPrompt).toHaveLength(1);
+		expect(request?.systemPrompt?.[0]).toContain("Generate a concise, sentence-case title");
+	});
+
+	it("uses the resolved TITLE_SYSTEM.md prompt for online title generation", async () => {
+		const model = getModelOrThrow("claude-sonnet-4-5");
+		const customPrompt = "Generate lowercase colon-delimited session names.";
+		const completeSimpleMock = vi.spyOn(ai, "completeSimple").mockResolvedValue({
+			stopReason: "stop",
+			content: [{ type: "toolCall", id: "call-title", name: "set_title", arguments: { title: "fix:resolver" } }],
+		} as never);
+
+		await generateSessionTitle(
+			"Investigate the resolver",
+			createRegistry(model),
+			createSettings(model),
+			undefined,
+			undefined,
+			undefined,
+			customPrompt,
+		);
+
+		const request = completeSimpleMock.mock.calls[0]?.[1] as
+			| { systemPrompt?: string[]; tools?: Array<{ name?: string }> }
+			| undefined;
+		const options = completeSimpleMock.mock.calls[0]?.[2] as
+			| { toolChoice?: { type?: string; name?: string } }
+			| undefined;
+		expect(request?.systemPrompt).toEqual([customPrompt]);
+		expect(request?.tools?.[0]?.name).toBe("set_title");
+		expect(options?.toolChoice).toEqual({ type: "tool", name: "set_title" });
+	});
+
 	it("falls back to text content when no set_title tool call is returned", async () => {
 		const model = getModelOrThrow("claude-sonnet-4-5");
 		vi.spyOn(ai, "completeSimple").mockResolvedValue({