fix(sessions): give derived Remote Control names a real, useful host suffix

[defangdevs]

Problem

Auto-derived Remote Control session names are <user>[-<session>]@<host>. Two bugs made <host> useless:

1. Dangling @ — <host> came from config.networking.fqdnOrHostName, evaluated at build time. On the AWS image networking.hostName is unset, so that string is empty and the name rendered as the literal agent@ (bare trailing @).
2. Internal fqdn — the empty-host fallback used the live kernel hostname, which on a cloud box is the internal, non-routable name (e.g. agent-devs@ip-10-42-1-151.eu-central-1.compute.internal) — no use for identifying or reaching the box in the Claude apps.

Changes

• Add services.agent-box.remoteControlHost (defaults to fqdnOrHostName). The AWS template sets it to the CloudFormation stack name, so boxes — and runtime-added sessions, which get remoteControlName: null — are identifiable as <user>[-<session>]@<stack>.
• When no explicit host is configured, fall back to the box's public web.domain (sslip.io on AWS, custom DNS on bare metal) — the address you actually reach the box at — before the live kernel hostname. The kernel hostname remains only as a last-resort guard so a name is never a dangling @.

Net effect: a box with no explicit remoteControlHost is now named after its public address (agent-devs@18.196.53.37.sslip.io) instead of an internal EC2 fqdn.

Testing

• tests/sessions.nix: with remoteControlHost unset, asserts the supervisor bakes the public web.domain into the derived name.
• Verified the module evaluates with remoteControlHost="" + web enabled (the lazy web.enable && … guard avoids reading the defaultless web.domain when web is off), and that the generated supervisor script bakes host=box.test.

🤖 Generated with Claude Code