docs: rewrite the README status notice and fix the opifex URL casing

[mahdi-shafiei]

docs: launch-readiness pass on the DiffBio README

What

Two changes, both required by decisions/0006-public-launch-plan.md before launch:

1. Reframes the status banner. The warning stays honest; the call to action inverts.
2. Removes the prose tells that taste-skill sections 9.D and 9.G ban from published copy.
   This file carried 3 em-dashes.

Why the banner changes

Every one of the six launch repos carried a banner that told visitors not to engage. The APIs
genuinely are unstable, so deleting the warning was considered and rejected: it is the one change
here that could actually cost credibility rather than stars. Instead the same information is
stated with an invitation attached, which is the higher-converting frame for a pre-1.0 project and
is also simply more accurate about what we want.

Nothing about maturity is softened. Pre-1.0 status, pinning advice, and the not-for-production
warning all remain.

Repo-specific notes

• Removes "We recommend waiting for a stable release (v1.0) before using DiffBio in production".
• Keeps the API / Tests / Documentation status table.
• Fixes the link to github.com/avitai/Opifex (capital O) to the canonical lowercase opifex.

Verification

python3 scripts/check_published_prose.py passes on this file: zero em-dashes, zero en-dashes,
zero banned verbs, middle dots within limit. The replacements were spot-checked by hand rather
than trusted to the transform: every site was a bullet-label separator where a regular hyphen
reads naturally.