feat(ai,agent,coding-agent): add sessionId for provider session-based caching

- Add sessionId to StreamOptions for providers that support session-based caching
- OpenAI Codex provider uses sessionId for prompt_cache_key and routing headers
- Agent class now accepts and forwards sessionId to stream functions
- coding-agent passes session ID from SessionManager and updates on session changes
- Update ai package README with table of contents, OpenAI Codex OAuth docs, and env vars table
- Increase Codex instructions cache TTL from 15 minutes to 24 hours
- Add tests for sessionId forwarding in ai and agent packages

packages/agent/test/agent.test.ts

@@ -229,4 +229,30 @@ describe("Agent", () => {
 		agent.abort();
 		await firstPrompt.catch(() => {});
 	});
+
+	it("forwards sessionId to streamFn options", async () => {
+		let receivedSessionId: string | undefined;
+		const agent = new Agent({
+			sessionId: "session-abc",
+			streamFn: (_model, _context, options) => {
+				receivedSessionId = options?.sessionId;
+				const stream = new MockAssistantStream();
+				queueMicrotask(() => {
+					const message = createAssistantMessage("ok");
+					stream.push({ type: "done", reason: "stop", message });
+				});
+				return stream;
+			},
+		});
+
+		await agent.prompt("hello");
+		expect(receivedSessionId).toBe("session-abc");
+
+		// Test setter
+		agent.sessionId = "session-def";
+		expect(agent.sessionId).toBe("session-def");
+
+		await agent.prompt("hello again");
+		expect(receivedSessionId).toBe("session-def");
+	});
 });