docs: note sibling rl-harness symlink for experiment smoke runs

[Al-does]

Summary

While setting up the Cloud Agent dev environment for this library, I hit one non-obvious gotcha and documented it in AGENTS.md under ## Cursor Cloud specific instructions.

The experiment repo (alex-rl-experiments) declares an editable dependency rl-harness = { path = "../rl-harness" }, but the cloud checkout of this library is RL-Harness (Linux is case-sensitive) and bootstrap_local.sh only auto-links RL Harness / rl-harness-src. So uv sync / smoke runs from the experiment repo fail until a sibling rl-harness -> RL-Harness symlink exists. This PR records the one-time sudo ln -sfn ... fix for future agents. The library's own uv sync --group dev needs no symlink.

Environment verification (no code change)

• uv sync --group dev (library) — provisions Python 3.14 into .venv, installs ray/torch/etc.
• uv run pytest -q -m "not slow" — 138 passed, 5 deselected.
• uv sync --group dev (experiment repo, via sibling symlink) — editable rl-harness resolves.
• uv run rl-harness experiments.mess3_belief_geometry_2026_07.reward_only.experiment --smoke — PPO trial trained 2 iterations on HMMEnv, terminated completed, episode_return_mean ≈ 41.55, run manifest written with full provenance.

Only AGENTS.md changes; no source code touched.