Implement agent versioning name@version

[nficano]

ARCP v1.1 §7.5 allows agent identifiers with version suffixes. Not implemented.

Steps:

1. Add AgentRef::parse('name@version') parser
2. Extend welcome capability shape with { name, versions: [...] }
3. Wire name@version into job.submit agent resolution
4. Add AGENT_VERSION_NOT_AVAILABLE error code (see related issue)
5. Add samples/agent_versions/ demo
6. Tests

Reference: spec §7.5.

---

Audit reference: ARCP SDK Audit v1.1 (2026-05-19)