Join Machines and use Entry Machine fallback through shared sessions #880

Parent

#868

What to build

Cloud joins another Machine through existing Machine RPC Register and uses preferred-entry fallback for existing RPC workflows. Enrollment and runtime share connection establishment instead of relay-specific discovery and transport fanout.

Acceptance criteria

• Use authorized candidates from the current pairing, preserve preferred ordering and bounded connection-establishment fallback, and call the SDK connection interface once per operation/session. Register remains Machine RPC, not Relay Register.
• Choose and confirm an entry before registration or another mutation; disconnection after dispatch reports uncertainty rather than replaying against a different entry.
• Delete EnrollmentRelay/interface/live wiring, held-state classification and Register Path RTT, relay-list transformations, top-level dial-and-register wrappers and duplicate attempt loops once callers migrate. Do not rename them to Tailcat equivalents.
• Verify existing Cloud read and mutation workflows plus streamed exec and remote Build upload on participating Machines. Preserve SSH/local auxiliary paths without inventing Tailcat proxy parity.
• Rung 2 — joining/registration, preferred fallback, no candidates versus unreachable, no mutation replay and stream cancellation through existing runtime/session interfaces. Rung 4 only for participating-Machine streaming behavior that lower rungs cannot prove.

Blocked by

• Enroll a Founding Machine and open it from Cloud through Tailcat #879