docs: scrub editorializing from README (v0.2.3)

[MarouaBoud]

A pass over the README to cut editorializing and self-praise while keeping every factual claim. Examples:

• "No LLM in the hot path, ever — that's the moat, not a footnote" → "No LLM in the response path."
• "mockworld takes the opposite bet —" → "mockworld is deterministic, MCP-native, and open:"
• "Legibility is a measurable property." → cut (the 32.5%→45.5% number makes the point)
• "determinism is non-negotiable" → "determinism is required"; "fast and pure" → "dependency-free"; dropped "high-fidelity", "instantly", "one command" filler.

Sibling-project blurbs in the toolkit table left as-is. Docs-only 0.2.3 so the PyPI page matches; twine check passes.