[Docs] Document environment variables and internal-* SLURM configs in the README

[aaronsmulktis]

What

Two new README sections for people cloning the repo to run OpenApps on a cluster, plus one stale-example fix.

Environment variables

• The .env / load_dotenv() path (both launch_agent.py and launch_parallel_agents.py call it).
• Table of the vars actually read by configs and code: USER (W&B entity + logs_dir), GPT55_API_KEY, the AWS trio for claude_4_sonnet (client_type: aws), WANDB_API_KEY / WANDB_BASE_URL / WANDB_MODE, EXPERIMENT_CONFIG_PATH, and OPENAPPS_APP / OPENAPPS_MCP_HOST / OPENAPPS_MCP_PORT.
• How to point an agent's api_key at whatever variable you use, via Hydra interpolation.
• The env knobs for scripts/conduct.sh (AGENTS, COUNT, MAX_PARALLEL, HEADLESS, LOG_DIR, WANDB_GROUP) and scripts/conduct_slurm.sh (VLLM_MODEL, VLLM_PORT, VLLM_HOST).

Running on SLURM

• config/mode/slurm_cluster.yaml ships placeholders that sbatch rejects. Documents the existing gitignored internal-* convention: copy it to config/mode/internal-slurm_cluster.yaml, fill in real paths/account/QOS, and select it with mode=internal-slurm_cluster. Includes a placeholder-only example of that file.
• vLLM host wiring (agent.hostname / agent.port), what conduct_slurm.sh discovery does, and overriding account/QOS/partition at submit time rather than editing the #SBATCH lines.

Drive-by fix: the quickstart used agent=GPT-5-1, which was renamed to the GPT-5.5-* configs — the command as written fails to resolve in Hydra. Now agent=GPT-5.5-computer-use with the matching GPT55_API_KEY. Happy to split this out if preferred.

Test plan

Docs only — no code changes. Claims verified against the repo:

# api_key interpolation override resolves
GPT55_API_KEY=x OPENAI_API_KEY=testkey uv run launch_agent.py \
  agent=GPT-5.5-computer-use 'agent.api_key=${oc.env:OPENAI_API_KEY}' --cfg job --resolve
#   api_key: testkey

# internal-* mode composes
uv run launch_parallel_agents.py mode=internal-slurm_cluster agent=dummy --cfg job
#   cluster: slurm, logs_dir/slurm_account from the untracked file

No cluster paths, hostnames, account names, or QOS names in the added text — placeholders only.