Polish OWD Sync product story, installation, and safety boundary

[msinclair25]

What changed

• give OWD Sync a stronger public product opening and private-alpha CTA
• explain the explicit vault connection with a visual flow and a concise
  does/never-does trust-boundary table
• make OWD Platform's version-matched desktop installer the primary alpha path
• keep BRAT as the disclosed unsupported-browser fallback
• document the exact pairing flow, unrecognized URI action repair,
  compatibility boundary, and separation from agent/Project authorization
• align the package description with the public product position

Why

The public plugin repository is often the first OWD code a prospective user can
see. It should explain why the companion exists and why its narrow authority is
a feature before asking the reader to follow installation steps.

The README now aligns with MD Evolved and OWD Platform: keep the vault as the
source, connect only what the owner approves, and keep agent authorization
separate from local synchronization.

Validation

• Prettier check passed for README.md and package.json
• package.json parsed as valid JSON
• every local Markdown link and image target exists
• git diff --check