docs(mcp): expand first-class client integration guides

[msaad00]

Summary

• add a major-client MCP guide that explains when to use agent-bom mcp server, agent-bom proxy, and agent-bom proxy-configure
• add a dedicated Codex CLI integration guide covering TOML config and manual proxy wrapping
• tighten the existing Claude, Cortex, README, MCP server, and product brief references so the MCP integration story is coherent across current docs

Why

The MCP surface is already wired in code, but the adoption gap is still documentation friction. The discovery engine already knows the real config paths for Claude, Cursor, Windsurf, Cortex, Codex, and Gemini. This patch turns that into first-class setup guidance instead of scattering the story across a few partial docs.

What changed

• new docs/MCP_CLIENT_GUIDES.md
• new docs/CODEX_CLI.md
• updated docs/CLAUDE_INTEGRATION.md
• updated docs/CORTEX_CODE.md
• updated docs/MCP_SERVER.md
• updated README.md
• updated docs/PRODUCT_BRIEF.md

Impact

• clearer per-client config paths across macOS, Linux, and Windows
• explicit guidance on MCP server mode vs runtime proxy mode
• Codex CLI is now a first-class documented path instead of an implied discovery surface
• current docs now point to one major-client matrix instead of making users stitch the path together themselves

Validation

• docs-only patch
• content grounded in the current discovery paths from src/agent_bom/discovery/__init__.py and src/agent_bom/discovery/config_parsers.py