Add unit tests for session memory module

[osisdie]

Overview

The lib/sessions/ module handles short-term memory (STM) and long-term memory (LTM) for all broker channels but currently has no test coverage.

What to do

Write tests using Bun's built-in test runner (bun test) covering:

• loadConfig() — config loading and defaults
• appendMessage() — STM message append and JSONL format
• buildContextPrompt() — context window construction from STM + LTM
• /session command parsing (status, clear, profile, forget, export)

Getting started

# Run tests
bun test

# Target files
lib/sessions/

Acceptance criteria

• Tests pass with bun test
• Cover at least the 4 functions listed above
• No external service dependencies (mock file I/O if needed)