Shared prompt surfaced to humans

What changed

• Documented the shared agent prompt location in README.md under "Agent Shared Prompt".
• Clarified that both Codex CLI and the Python lighthouse_agent.py read instructions from prompts/lighthouse-shared.md.

Why it matters

• Makes it obvious to humans where to edit the working rhythm and tool list for both agents in one place.
• Reduces the risk of prompt drift between Codex CLI and the Python agent as the culture and tooling evolve.

Next ideas

• Optionally add a short note in HANDOFF.md pointing to prompts/lighthouse-shared.md for deeper prompt tweaks.