Make README usage path more concrete

[klmtseng]

Improves the README landing experience for first-time users without changing runtime or protocol behavior.

Changes:

• replaces the abstract opening tagline with a more concrete description of Validity Audit as a fail-closed audit layer
• adds a When would I use this? section immediately after the architecture diagram
• shows where Validity Audit sits in a typical coding-agent workflow
• states explicitly that it complements rather than replaces pytest, ruff, npm test, CI, or a done-gate
• adds two concrete fail-open examples to make the verifier-challenge idea easier to understand

No runtime, schema, policy, benchmark, release-tag, or v0.4.0 artifact changes.