docs: add contextual launch pattern

[Chipagosfinest]

What changed

• document tool arguments as the portable way to pass app-specific launch data
• add type-checked server and View examples for multiple contextual entry points
• distinguish launch input from authenticated identity or trusted host provenance
• document the current hostContext.toolInfo limitation and link issue Provide a consistent way for MCP Apps to know which tool invoked them #492

Why

Telegram and Farcaster make contextual launches useful through small entry parameters, but MCP Apps already has the necessary transport: tool arguments arrive through ui/notifications/tool-input. Documenting that mapping gives builders a portable pattern without adding a new protocol primitive.

The security guidance recommends bounded inputs and opaque server-side handles, and warns against putting credentials, raw conversation content, or personal data in launch arguments.

Prototype validation

The pattern is now exercised by Discussion Capsule, a standalone read-only MCP App. Its open_github_discussion tool accepts a Discussion URL as an ordinary argument and returns source-grounded structuredContent for the linked view.

I tested it against ext-apps Discussion #606 through this repository's basic reference host:

• the real Discussion loaded with 4 comments, 4 participants, and 8 source-linked questions
• the app rendered through the sandbox with zero browser console errors
• host-message handoff, comment filters, source links, and draft selection worked
• the app never posts or mutates GitHub state

This prototype does not require a separate launch envelope or claim that tool input is authenticated provenance.

Validation

• npm run prettier
• npm test — 374 passed, 1 skipped
• npm exec typedoc -- --treatValidationWarningsAsErrors --emit none
• pre-commit npm run build:all and npm run prettier:fix

Prototype repository:

• npm test — 7 passed
• npm run build
• live GitHub REST smoke test against Discussion Proposal: JSON-LD Web Markup for MCP App Discovery #606
• Playwright interaction pass through the ext-apps basic host

AI assistance disclosure

This PR and the linked prototype were researched and authored with Codex under Alec Gutman's direction. The contribution uses the existing MCP Apps tool-input lifecycle and adds documentation plus type-checked examples; it does not change the protocol or runtime.