feat: implement ssh_remote and colab_drive experiment modes

[Pseudoataraxia]

Summary

• Implement ssh_remote experiment mode: execute experiments on remote GPU servers via SSH, with optional Docker-over-SSH for full container isolation
• Implement colab_drive experiment mode: async execution via Google Drive sync with a Colab worker notebook — no SSH tunnel needed
• Fix bug where llm.acp.timeout_sec was parsed from config but never passed to ACPClient, causing all ACP calls to use the hardcoded 600s default

Details

ssh_remote mode

• New SshRemoteSandbox class implementing SandboxProtocol
• Two execution modes: bare Python (with HOME override + unshare --net sandboxing) or Docker-over-SSH (full container isolation)
• Extended SshRemoteConfig with user, port, key_path, remote_python, setup_commands, and Docker options
• Explicit WARNING when unshare is unavailable instead of silent fallback

colab_drive mode

• New ColabDriveSandbox class implementing SandboxProtocol
• Protocol: pipeline writes to pending/, Colab worker polls and executes, results written to done/
• Includes ready-to-use Colab worker notebook template (auto-generated on first run)
• Configurable poll interval and timeout for Google Drive sync latency

ACP timeout fix

• ACPClient.from_rc_config() was not passing timeout_sec from config, so all ACP calls used the hardcoded 600s default regardless of config

Test plan

• 28 new unit tests covering command building, connectivity checks, mocked execution, factory integration, and ACP timeout fix
• All 1251 existing tests pass (no regressions)
• End-to-end verified: SSH sandbox on macOS localhost with real ssh + scp
• End-to-end verified: Colab Drive sandbox with simulated worker (submit → poll → collect)
• Not yet tested on a real remote GPU server or real Google Colab instance

Config examples

# SSH remote (bare Python)
experiment:
  mode: "ssh_remote"
  ssh_remote:
    host: "gpu-server.lab.edu"
    user: "researcher"
    key_path: "~/.ssh/id_rsa"
    gpu_ids: [0]

# SSH remote (Docker-over-SSH, most secure)
experiment:
  mode: "ssh_remote"
  ssh_remote:
    host: "gpu-server.lab.edu"
    user: "researcher"
    use_docker: true
    docker_network_policy: "none"

# Colab via Google Drive (no SSH needed)
experiment:
  mode: "colab_drive"
  colab_drive:
    drive_root: "~/Library/CloudStorage/GoogleDrive-you@gmail.com/My Drive/researchclaw"
    poll_interval_sec: 30
    timeout_sec: 3600

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