chore(runtime-host): retire the legacy surface hello shim after epoch 27

[Sun-GLiang]

Summary

PR #3277 removes ClientSurface from the Runtime Host protocol and connection plumbing, while retaining one private outgoing hello shim that sends surface: "desktop" for pre-#3277 Hosts.

Track removal of that shim once the compatibility boundary is observable on the wire.

Removal gate

Remove the shim only when all of the following are true:

• the minimum supported Runtime Host compatibility epoch is greater than 27;
• the removal change bumps the Client compatibility epoch accordingly;
• mixed-version coverage verifies that unsupported older Hosts reach the structured incompatible / restart-guidance path rather than failing with a bare transport abort.

Epoch 27 cannot distinguish a pre-#3277 Host that requires surface from a post-#3277 Host that tolerates its absence, so release notes alone are not a safe removal signal.

Related

• Follow-up to chore(runtime-host): ClientSurface is a required handshake field with zero readers #3216
• Compatibility shim introduced by refactor(runtime-host): retire client surface identity plumbing #3277
• Released-Host compatibility guard added by test(runtime-host): guard v0.1.11 handshake compatibility #3322

AI assistance disclosure: Codex helped analyze the mixed-version failure mode and draft this tracking issue.

[Sun-GLiang]

I’ll take this follow-up next. After #3277 lands, I’ll implement the epoch-gated retirement path and add mixed-version coverage proving that unsupported epoch-27 Hosts receive actionable incompatibility/restart guidance rather than a bare transport abort.