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/ai/src/providers/openai-codex-responses.ts

@@ -96,6 +96,7 @@ export const streamOpenAICodexResponses: StreamFunction<"openai-codex-responses"
 				model: model.id,
 				input: messages,
 				stream: true,
+				prompt_cache_key: options?.sessionId,
 			};
 
 			if (options?.maxTokens) {
@@ -132,7 +133,7 @@ export const streamOpenAICodexResponses: StreamFunction<"openai-codex-responses"
 			const transformedBody = await transformRequestBody(params, codexOptions, systemPrompt);
 
 			const reasoningEffort = transformedBody.reasoning?.effort ?? null;
-			const headers = createCodexHeaders(model.headers, accountId, apiKey, transformedBody.prompt_cache_key);
+			const headers = createCodexHeaders(model.headers, accountId, apiKey, options?.sessionId);
 			logCodexDebug("codex request", {
 				url,
 				model: params.model,