feat(mcp): implement mcp-hub for hybrid host-cluster visibility

[victoriacheng15]

Summary

Establishes the "Hybrid Intelligence" layer by implementing mcp-hub, a dedicated Model Context Protocol server for host-level introspection. This update bridges the visibility gap between Kubernetes orchestration and physical host services, providing AI agents with namespaced tools for resource analysis and direct journal retrieval.

List of Changes

• Agentic Host Control: Implemented mcp-hub server with namespaced tools (hub_) to provide surgical visibility into host-resident systemd services (Ingestion, Proxy, OpenBao) and physical resource pressure.
• Introspection Abstraction: Engineered a CommandRunner interface to enable deterministic parsing of OS-level state, verified with 100% table-driven unit test coverage.
• Service Registry Integration: Unified the MCP tool registry to support cross-domain investigation by aligning the hub's perceptual organs with the platform's established implementation patterns.
• Strategic Documentation: Formalized the design rationale in ADR 019 and synchronized the platform's evolution timeline to reflect the completion of the hybrid intelligence bridge.

Verification

• make test passes for internal/mcp verifying both provider logic and tool handlers.
• mcp-hub build verified via make mcp-hub-build.
• Manual verification of namespaced tool registration (hub_inspect_platform, etc.) in registry logic.