client context should guide agents to use mounted sandbox workspace paths

[chubes4]

Problem

Homeboy Codebox multi-cell dispatch now succeeds at the orchestration/runtime layer, but sandbox agents can still answer poorly because Data Machine renders client_context.sandbox_workspace and client_context.default_workspace as raw JSON without explicit operational guidance.

Evidence

Run id:

proof-homeboy-multicell-stable-main-20260607-001

The run succeeded with two concurrent Codebox cells, both with zero changed files and empty patches, but one cell answered:

/workspace directory exists.

Cannot find a repository named "repo" in /workspace for git branch/status summary.

Please specify the exact repository name present in /workspace ...

The runtime input included a concrete mounted workspace contract:

"client_context": {
  "workspace_root": "/workspace",
  "sandbox_workspace": {
    "root": "/workspace",
    "defaultMode": "repo-backed",
    "mounts": [
      {
        "target": "/workspace",
        "mode": "readwrite",
        "sourceMode": "repo-backed",
        "mountRole": "homeboy-dmc-workspace"
      }
    ]
  },
  "default_workspace": {
    "target": "/workspace",
    "mode": "readwrite",
    "sourceMode": "repo-backed",
    "mountRole": "homeboy-dmc-workspace"
  }
}

But the directive only renders this as generic current client context, so the model guessed a Data Machine workspace handle named repo instead of treating /workspace as the mounted filesystem root.

Expected behavior

When client_context.sandbox_workspace or client_context.default_workspace is present, Data Machine should add explicit system guidance that:

• /workspace is the mounted sandbox repository root.
• The agent should treat the default_workspace.target as the current repo root.
• It should not invent DMC workspace handles such as repo for mounted workspace inspection.
• If using workspace tools, it should prefer the mounted workspace path/root supplied by client context.

Acceptance criteria

• ClientContextDirective emits a dedicated sandbox workspace guidance block when sandbox workspace context is present.
• Existing editor context guidance continues to work.
• Unit/smoke coverage verifies the guidance includes the default workspace target and handle-warning text.
• A Homeboy Codebox read-only fanout cell can answer the workspace inspection task without asking for a made-up repo handle.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Filed from the Homeboy Codebox multi-cell proof run and runtime transcript evidence.