Use canonical cast.agentrelay.com Relay transport default throughout installed tree

[willwashburn]

Summary

The installed Relayflows dependency tree still defaults Relay transport to the legacy host https://api.relaycast.dev; https://cast.agentrelay.com is canonical.

Credit: found by cpo while inspecting the complete installed Agent Relay 11.3.1 tree, dependencies included. The installed bundle contains eight legacy-host occurrences, including a nested @relaycast/sdk. Relayflows source currently has two fallback literals in packages/core/src/runner.ts, so correcting only those literals is not enough if the resolved SDK dependency still embeds the legacy default.

Required fix

• Replace Relayflows-owned fallback defaults with https://cast.agentrelay.com.
• Update or constrain the dependency graph so nested @relaycast/sdk code does not restore the legacy default.
• Add tests covering the effective default when no base-URL environment variable or explicit option is supplied.
• Verify the built package and an installed consumer tree, dependencies included, contain no reachable api.relaycast.dev default.

Release note

Fix and publish are separate operations. Do not publish packages or create a release from this issue without the release owner's approval.