test: add unit tests for AgentInstanceManager getOrCreate identity semantics (Issue #113)

[rafaio1]

Summary

Adds unit tests verifying AgentInstanceManager.getOrCreate identity semantics including reference stability, createdAt preservation, distinct instances for different IDs, and empty-ID rejection.

Tests Added

• ✅ Same instance reference returned for identical agentId
• ✅ Original createdAt preserved on subsequent calls
• ✅ Distinct instances created for different agentIds
• ✅ agentId and ISO createdAt populated on creation
• ✅ list() returns all instances without duplicates
• ✅ Empty agentId rejected with INVALID_TASK error

Verification

All 6 tests pass locally via npx vitest run tests/agents/agent-instance-manager.test.ts.

Closes #113