docs: add a glossary for route, pack, guard, MCP, and benchmark

[vishal2612200]

Goal

Add a small glossary for common AgentPack terms used across the README and docs.

Suggested files

• docs/index.md or docs/how-agentpack-works.md

Acceptance criteria

• Defines route, pack, guard, MCP, benchmark, context pack, and freshness.
• Links to the full command docs where useful.
• Uses plain language for first-time contributors.