Add BUDGET_EXHAUSTED, AGENT_VERSION_NOT_AVAILABLE error codes

[nficano]

ARCP v1.1 §12 adds three new error codes. Kotlin SDK already has LEASE_EXPIRED. Missing: BUDGET_EXHAUSTED, AGENT_VERSION_NOT_AVAILABLE.

Steps:

1. Add both to the error code enum/sealed class in lib/src/main/kotlin/dev/arcp/errors/
2. Wire BUDGET_EXHAUSTED into cost.budget enforcement path
3. Wire AGENT_VERSION_NOT_AVAILABLE into agent resolution path (when name@version not in welcome's agents advertisement)
4. Update conformance tests
5. Document in docs/

Reference: spec §12 error taxonomy.

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