feat(remote): add an instance type to remote deploys and fleet nodes

[outofcoffee]

A kind: remote node and a standalone remote deploy can now name the EC2 instance type the environment's instances launch as, instead of always using the control plane's default.

Summary

• remote deploy --instance-type and a fleet node's instance-type both record the type on the environment's deploy config.
• The environment's start launches with that type; without it, the control plane's default type is used.
• Naming instance-type on a kind: daemon node, or a malformed value anywhere, is rejected with the value named.
• The deploy plan prints the resolved type (or states the default), and the no-capacity reply names the type it was trying.
• OpenAPI schema, the fleet.yaml and remote deploy reference docs, and the fleet-remote example are updated.

Implementation details

The type is stored on the environment's per-environment deploy config (the SSM parameter the start Lambda reads on each wake) and read back on the next fresh launch. A re-wake of a stopped instance keeps the type it was launched with — EC2 cannot resize an existing instance — so a changed value takes effect only after the instance is terminated (an idle sweep or remote stop) and launched again. The field is optional and omitted when empty, so an untyped deploy sends exactly what an older control plane expects.

Closes #192