sandbox: smolmachines backend (#478)

Adds smolmachines (api.smolmachines.com) as a sandbox backend: a scope's agent computer runs on a smolmachines microVM, selected per scope through the existing sandbox router and migration machinery. One persistent machine per scope with the same name-slug scheme as other backends; the default image is the registry's coding-agent image (Debian with git/curl/python3/node/jq). Machines are created stopped so create explicitly starts and polls; a platform-stopped machine is restarted and the exec retried once, and a 409 name conflict on create (blue-green race) adopts the existing machine. Exec, file transfer, process sessions, backup, and blob staging are implemented against a test fake, and the dev launcher gains a -- sandbox smolmachines flag.

**Deployment notes**
Entirely opt-in new sandbox backend gated on SMOLMACHINES_TOKEN (plus optional
BASE_URL/NAME_PREFIX/IMAGE/EGRESS_PROXY_URL); no schema changes; CLI secret spec
added for schema parity
Blue-green race on machine create handled explicitly: 409 name conflict adopts the existing
machine

<!-- codesmith:footer -->
---
<a href="https://app.blacksmith.sh/yc-software/codesmith/qm/pr/478"><picture><source media="(prefers-color-scheme: dark)" srcset="https://pr-comments-assets.blacksmith.sh/codesmith/view-with-codesmith-dark-v2.svg"><source media="(prefers-color-scheme: light)" srcset="https://pr-comments-assets.blacksmith.sh/codesmith/view-with-codesmith-light-v2.svg"><img alt="View with [code]smith" src="https://pr-comments-assets.blacksmith.sh/codesmith/view-with-codesmith-dark-v2.svg"></picture></a> <a href="https://backend.blacksmith.sh/track/enable-autofix?expires=1789249012&installation_model_id=19911&pr_number=478&repository=yc-software%2Fqm&return_to=https%3A%2F%2Fgithub.com%2Fyc-software%2Fqm%2Fpull%2F478&signature=d284d44e394b485d544ff5fe5c1c506608b3eaa9e0b74554495d1696aad0f5b1"><picture><source media="(prefers-color-scheme: dark)" srcset="https://pr-comments-assets.blacksmith.sh/codesmith/autofix-with-codesmith-dark.svg"><source media="(prefers-color-scheme: light)" srcset="https://pr-comments-assets.blacksmith.sh/codesmith/autofix-with-codesmith-light.svg"><img alt="Autofix with [code]smith" src="https://pr-comments-assets.blacksmith.sh/codesmith/autofix-with-codesmith-dark.svg"></picture></a>
<sup>Need help on this PR? Tag <code>@codesmith-bot</code> with what you need. Autofix is disabled.</sup>

<!-- codesmith:autofix:disabled -->
<!-- /codesmith:footer -->