docs: present Cortex as a cross-platform memory MCP

[cdeust]

Summary

• lead with Cortex as a cross-platform memory MCP for Codex, Gemini CLI, Claude Code, and other local stdio MCP hosts
• align README, MCPB, npm, PyPI, and MCP Registry descriptions and keywords
• keep the boundary explicit: the 52-tool server is host-agnostic, while automatic lifecycle capture and injection remain Claude Code plugin hooks

The GitHub repository description and topics were updated separately so the same positioning is visible in search and discovery.

Validation

• python scripts/check_doc_claims.py
• pytest --confcutdir=tests_py/scripts -q tests_py/scripts/test_cross_host_manifests.py tests_py/scripts/test_codex_plugin_contract.py tests_py/scripts/test_check_doc_claims.py — 116 passed
• git diff --check