refactor(relaycast): consume sdk bootstrap helpers

[noodleonthecape]

Summary

• switch relay's broker-side Relaycast bootstrap logic to the new SDK helpers
• use the Rust SDK's idempotent workspace-stream and channel ensure/join APIs instead of hand-rolled 409 handling in relay
• pin the Rust dependency to the relaycast SDK branch commit so this PR remains buildable before a crates.io release

Why

Relay had SDK-shaped logic locally in src/relaycast_ws.rs for:

• enabling the workspace stream before websocket connect
• creating broker channels if missing
• joining those channels while treating create/join conflicts as no-ops

That logic now lives in the relaycast SDK, so relay should consume it rather than re-implement it.

Validation

• Cross-repo validation completed for the new SDK API surface in AgentWorkforce/relaycast:
  • npm run build --workspace=@relaycast/types
  • npx vitest run packages/sdk-typescript/src/__tests__/agent.test.ts packages/sdk-typescript/src/__tests__/relay.test.ts
• Relay-side Rust tests could not be executed in this environment because cargo is not installed on the host.
• The relay Cargo.lock entry was updated to the exact relaycast git revision used by this PR so CI can resolve the dependency consistently.

Depends on

• feat(sdk): add idempotent workspace bootstrap helpers relaycast#111

---

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.