feat(retained): expose continuation control on admission

[drewstone]

Summary

• return the exact continued-run control reference after durable admission and before terminal output
• preserve one idempotent operation across admission retry, terminal replay, restart, stream, status, and cancellation
• advertise the capability only with the released Agent Interface 1.8.0 contract
• repair successful version probes for runners such as Prime Agent that write their version to stderr

Proof

• pnpm typecheck
• retained-session suite: 116 passed
• full suite: 1,022 passed and 11 skipped across 1,033 tests
• real installed prime-agent 0.7.0 health probe passed
• merge-tree against origin/main passed

The matching consumer contract shipped in tangle-network/agent-sdk#252.