feat(runs): bind exact retained coordinates

[drewstone]

Summary

• Preserve caller-owned provider, environment, session, and execution coordinates on durable runs.
• Return all six identity fields on dispatch, status, replay, and cancellation responses.
• Advertise the generic durable contract only after the selected backend passes its health check.
• Validate each coordinate with the Agent Interface field limit.

Proof

• pnpm verify
• 24 durable-run tests passed.
• 113 retained-session tests passed.
• Agent SDK provider integration passed 2 of 2 tests against this checkout.
• A 227-character Braid environment ID passed dispatch and status recovery.
• git diff --check