feat: add per-thread system prompts

[AaronCQL]

Summary

• Add per-thread system prompt support via --append-system-prompt CLI flag
• Prompt files stored at ~/.miniclaw/data/prompts/{chatID}_{threadID}.md (or {chatID}.md for non-threaded chats)
• If a prompt file exists and is non-empty, its contents are appended to the system prompt alongside the global CLAUDE.md
• Threads without a prompt file are unaffected
• Auto-create data/prompts/ directory on startup

Test plan

• Create a prompt file for a thread and verify it's appended to the system prompt
• Verify threads without prompt files work as before
• Verify the agent can read/write the prompt file when asked to update the thread's system prompt