Agent registry: named agents with lazy instantiation

[JaimeStill]

Summary

Adds an agent registry to the agent package — named agent management with lazy instantiation, capability querying, and config-driven registration. The kernel creates and owns a registry instance, populating it from config during initialization.

Closes #24

Changes

• agent/registry.go — Registry type with Register, Replace, Get, List, Unregister, Capabilities
• agent/errors.go — sentinel errors for registry operations
• kernel/config.go — Agents map field for named agent configs
• kernel/kernel.go — registry wiring, Registry() accessor, WithRegistry option
• Tests: 15 registry tests + 4 config tests + 3 kernel integration tests
• Documentation updates across project context files