docs: document sbx mount/umount for running sandboxes

[dvdksn]

Documents sbx mount / sbx umount, which expose host paths into an
already-running sandbox at runtime — without stopping or recreating it — using
a docker run -v-style spec (HOST[:CONTAINER_TARGET[:ro|rw]]).

Upstream change: docker/sandboxes#3542

What's added

A new Mounting host paths into a running sandbox section in
content/manuals/ai/sandboxes/usage.md covering:

• Allowlist-only mounts (HOST), visible under /mnt/host/
• Bind mounts at an absolute container target (HOST:CONTAINER_TARGET)
• Read-only mounts (:ro, enforced at the kernel mount layer)
• Idempotency, the host-path-must-exist rule, and relative-path resolution
• Filesystem policy enforcement, cross-linked to the governance concepts page
• Revoking paths with sbx umount (and passing the target back to unbind)

Notes

• The sbx mount / sbx umount CLI reference pages
  (/reference/cli/sbx/mount/, .../umount/) are generated from the vendored
  data/sbx_cli/ YAML, which is synced from docker/sandboxes at release time.
  They'll appear automatically once the release that ships these commands is
  vendored, so this PR intentionally doesn't hand-add those files or link to
  the not-yet-generated pages.
• Opened as a draft to land alongside the release that ships the feature.

🤖 Generated with Claude Code

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[docker-agent]

Assessment: 🟢 APPROVE

The new Mounting host paths into a running sandbox section is clean, well-structured, and consistent with the existing document's terse, direct style. Code examples match the described behavior, the relative link to governance/concepts.md#filesystem-rules resolves correctly (the governance/ directory is a sibling of usage.md), and no hedge words, meta-commentary, or marketing language were found. The section fits the existing document's scope and length without inflating it.

No issues to report.

[mschfh]

Is this feature already GA, at least the current rc ships with the command?

[dvdksn]

hi @mschfh no this feature is not GA yet - the commands exist but they are still hidden while we work them out!