feat: expose memfs (memory filesystem) option in lettabot config and SDK session (#336)

Adds features.memfs config key that controls whether the Letta Code CLI
receives --memfs or --no-memfs when creating/resuming SDK sessions. This
enables lettabot users to opt into git-backed memory filesystem (context
repositories) for persistent local memory sync.

- Config types: memfs?: boolean on AgentConfig.features, LettaBotConfig.features, BotConfig
- Bot wiring: baseSessionOptions() and createAgent() pass memfs to SDK when defined
- Main wiring: YAML config takes precedence, LETTABOT_MEMFS env var as fallback
- Legacy fix: conversations passthrough in single-agent normalization
- Tests: 3 memfs wiring tests (true/false/undefined), 2 conversations passthrough tests
- Docs: configuration.md section with known limitations, example YAML

Fixes #335

Written by Cameron and Letta Code

"The best way to predict the future is to implement it." -- David Heinemeier Hansson

lettabot.example.yaml

@@ -73,6 +73,7 @@ features:
     enabled: false
     intervalMin: 30
     # skipRecentUserMin: 5   # Skip auto-heartbeats for N minutes after user message (0 disables)
+  # memfs: true   # Enable memory filesystem (git-backed context repository). Syncs memory blocks to local files.
 
 # Attachment handling (defaults to 20MB if omitted)
 # attachments: