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/coding-agent/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Added
+
+- Session ID is now forwarded to LLM providers for session-based caching (used by OpenAI Codex for prompt caching).
+
 ### Fixed
 
 - Add `minimatch` as a direct dependency for explicit imports.

packages/coding-agent/src/core/agent-session.ts

@@ -856,6 +856,7 @@ export class AgentSession {
 		await this.abort();
 		this.agent.reset();
 		this.sessionManager.newSession(options);
+		this.agent.sessionId = this.sessionManager.getSessionId();
 		this._steeringMessages = [];
 		this._followUpMessages = [];
 		this._pendingNextTurnMessages = [];
@@ -1666,6 +1667,7 @@ export class AgentSession {
 
 		// Set new session
 		this.sessionManager.setSessionFile(sessionPath);
+		this.agent.sessionId = this.sessionManager.getSessionId();
 
 		// Reload messages
 		const sessionContext = this.sessionManager.buildSessionContext();
@@ -1745,6 +1747,7 @@ export class AgentSession {
 		} else {
 			this.sessionManager.createBranchedSession(selectedEntry.parentId);
 		}
+		this.agent.sessionId = this.sessionManager.getSessionId();
 
 		// Reload messages from entries (works for both file and in-memory mode)
 		const sessionContext = this.sessionManager.buildSessionContext();

packages/coding-agent/src/core/sdk.ts

@@ -613,6 +613,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			tools: activeToolsArray,
 		},
 		convertToLlm,
+		sessionId: sessionManager.getSessionId(),
 		transformContext: extensionRunner
 			? async (messages) => {
 					return extensionRunner.emitContext(messages);