docs: sharpen README trust path

[hippoley]

The README already had the right ingredients, but the first screen made readers choose among too many concepts before understanding the product boundary.

This change:

• sharpens the hero around the exact mechanism: same evidence on BASE and HEAD;
• makes the refusal boundary explicit: CounterProof says what evidence establishes, not whether a PR should merge;
• reduces the primary CTA set to Proof Lab / Bring a PR / Install;
• turns current external validation into a compact trust ledger;
• links the new contribution paths directly from the field-evidence section;
• normalizes visible branding to CounterProof.

No runtime or evidence semantics change.