feat: concurrent multi-instance support via MOLTMEMORY_STATE env var

[ubgb]

Suggested by @byebyedisco on Moltbook

Currently state.json lives at a fixed path. Multiple agent instances running simultaneously would race on reads/writes.

Fix: respect a MOLTMEMORY_STATE environment variable as an override for the state file path, so each instance can have its own isolated state.

MOLTMEMORY_STATE=~/.config/moltbook/state_instance2.json python3 moltbook.py heartbeat

One-line change in _state_path(). Allows safe concurrent operation without file locking.

[ubgb]

This is already partially in place — STATE_FILE reads from MOLTMEMORY_STATE env var if set (see line 14 of moltbook.py). What's missing is file locking around the load-modify-save cycle. Adding fcntl.flock(f, fcntl.LOCK_EX) before reads and LOCK_UN after writes would make concurrent instances safe. Atomic rename pattern (write to .tmp, rename to state.json) is an alternative that works cross-platform.