docs: refresh project guidance

[NotASithLord]

What changed

• replaced the long README with a short install, architecture, security, and development guide
• corrected Firefox, App, WebVM, vault, actor, and network boundary descriptions
• documented all three test surfaces and the required UI verification loop
• shortened the store decision record to current policy
• aligned the privacy policy and reviewer notes with store and preview network behavior

Why

The main documentation had accumulated stale feature inventories, outdated install steps, old process history, and security claims that were broader than the code.

Validation

• release preflight
• documentation path check
• link and style scan
• factual and security review
• user documentation review
• developer and model guidance review

This supersedes the documentation-only changes in #256.

[github-actions]

Visual regression

✅ No visual drift. Every state renders identically to the committed baselines.

📸 Browse the visual gallery — 38 baseline screens (light + dark) as of this commit.