Artifact sync now reads sandbox network-egress state from relay responses

What

• Artifact sync's status-reason handling gains a new relay_unavailable state (with a corresponding "latched" variant), alongside the existing no_originator/subscribe_forbidden states.
• A new x-ccr-sandbox-egress response header is now read from relay routes, with recognized values disabled, restricted, or unrestricted. The value is recorded per artifact (by slug) with a 10-minute freshness window, and is read back to decide whether sandbox network access should be reported as disabled for that artifact.

Why

This lets Claude Code reflect the actual, current sandbox network-egress restrictions for an artifact based on what the relay reports, and gives a clearer signal (relay_unavailable) when the relay itself can't be reached.