Implement agent versioning name@version

[nficano]

ARCP v1.1 §7.5 allows agent identifiers to carry a version suffix (agent-name@1.2.3), and session.welcome.payload.capabilities.agents may use a richer object shape advertising available versions. Not implemented.

Steps:

1. Add agent reference parsing (AgentRef.parse("name@version")) — handle bare names and name@version forms
2. Extend the welcome capability shape to support { name: string, versions: string[] }
3. Add AGENT_VERSION_NOT_AVAILABLE error code (see related issue)
4. Wire into job.submit agent resolution
5. Add samples/AgentVersions/ mirroring typescript-sdk/examples/agent-versions/
6. Add tests

Reference: spec §7.5, §13.7.

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