EXT: Ship extension SDK basics

[MikeSchirtzinger]

Problem

The extension isolation/security surface is stronger than the contributor experience. Third-party extension work needs typed contracts, manifest validation artifacts, and a mock host so contributors can build without reverse-engineering the app internals.

Evidence

• Extension host code and sandboxing exist, but there is no shipped silent-ext.d.ts contract for extension authors.
• Manifest validation is backed by Rust types, but no generated JSON Schema is published for manifest.json authoring.
• No mock-host test harness is available for extension development outside the full app.

Acceptance criteria

• Add silent-ext.d.ts covering host APIs, grant shapes, event envelopes, and UI/data boundaries.
• Generate or maintain a JSON Schema for extension manifest.json from the Rust manifest types.
• Provide a mock-host harness for local extension tests.
• Update extension docs with a minimal working extension, test command, and packaging expectations.
• Keep docs explicit that hosted deploys only enable extensions when the real extension route is available.