feat(readiness): probe dependencies with bounded backoff

[Yacobolo]

Summary

• probe SQLite, Swarm, capacity, CAS capabilities, and BuildKit through a bounded readiness path
• keep liveness independent from dependency readiness
• centralize exponential, jittered retry policy for heartbeat and Buildx flows

Test plan

• task test
• retry and readiness race tests

Linear: https://linear.app/leapstack/issue/LEA-290

Stack: 5/6 (codex/lea-289-worker-resource-envelopes → codex/lea-290-dependency-readiness-backoff)