Adopt Sandbox credential brokering when it ships #15

Context

Today the model API key reaches the investigation sandbox as a one-time file that the runner reads and deletes, and the runner is made non-dumpable so same-user tool commands can't read its memory. Git credentials are kept out of investigation sandboxes entirely by building snapshots in a separate builder sandbox and scrubbing them before the snapshot is taken.

This is careful, but it's still the template's own mechanism, and it leaves residual risk (the key is in process memory, and DNS or a bypassed shell policy could exfiltrate it).

Task

When Render Sandboxes ships credential brokering:

• Replace the one-time model key file with a brokered credential so the key never enters the sandbox.
• Evaluate brokering the git token in the builder sandbox, which could remove the scrub-and-verify step.
• Update docs/design.md, docs/sandboxes.md, the README security section, and the template page.

Blocked on the platform feature; keep this open as a tracking issue.