feat(json): support optional openDepth to expand nested containers

[benvinegar]

What

This PR adds the optional openDepth field to JsonSurface. It allows agents to specify how many levels of nested containers are expanded by default on initial render.

• openDepth: 0 (default): only the root container (object or array) is open.
• openDepth: 2: the root + first two nested levels of containers are open at startup.

Changes

• types.ts: Add optional openDepth: number to JsonSurface interface, and include it in surfacesByteLength calculations.
• postSurfaces.ts: Add openDepth to strict and loose zod validation schemas and transforms.
• JsonSurface.tsx: Pass openDepth recursively down, setting the default open signal state to depth <= openDepth instead of depth === 0.
• sideshow.js (CLI): Add --open-depth <n> to the json subcommand and the --json option of the publish command.
• DESIGN_GUIDE.md: Document openDepth for JSON surfaces and show CLI examples.
• api.test.ts / assets.test.ts: Add validation and round-trip tests.

Verification

All 351 tests pass. Linter, formatter, and typecheck are clean.