Write SANDBOX.md — formal sandbox and permissions spec

[omscse]

Document the formal sandbox model and permissions specification for .pweb bundles. This is a prerequisite for spec v1.0.

Should cover:

• Default-deny capability model
• The nine capability categories and their viewer tier requirements
• How capabilities are declared in manifest.json
• Runtime enforcement rules (what the viewer must block/allow)
• Origin isolation model (bundle:// synthetic origin)
• CSP injected by the viewer
• External link interception behaviour
• Storage scoping rules

See MANIFEST.md and the capability table in CLAUDE.md for the current draft.