feat: add memfs option to SDK session/agent options

[cpfiffer]

Summary

• Adds memfs?: boolean option to CreateSessionOptions, CreateAgentOptions, and InternalSessionOptions
• Passes --memfs / --no-memfs flags to the CLI subprocess in transport.ts
• Surfaces memfsEnabled?: boolean on SDKInitMessage so callers can confirm the state
• Companion PR: letta-ai/letta-code (surfaces memfs_enabled in the headless init message)

Usage

// Enable memfs on an existing agent
const session = createSession(agentId, { memfs: true });
const init = await session.initialize();
console.log(init.memfsEnabled); // true

// Or when creating a new agent
const agentId = await createAgent({ memfs: true });

// Disable memfs
const session = resumeSession(agentId, { memfs: false });

Test plan

• Verify tsc --noEmit passes (confirmed locally)
• Test memfs: true with a Letta Cloud agent -- session should initialize with memfs enabled
• Test memfs: false disables memfs
• Test memfsEnabled is returned on init message (requires companion letta-code PR)
• Test omitting memfs option leaves state unchanged

Written by Cameron and Letta Code

"Memory is the diary that we all carry about with us." -- Oscar Wilde