docs: add schema/types documentation with cross-linking

[rhuss]

Summary

Add a docs/src/schema/ section documenting SDK data types organized by domain concept (sandbox, policy, provider, etc.). Each page includes field tables, type relationships, and usage examples. All 16 existing API reference pages get inline cross-links from type names to schema pages.

Key Requirements

• New docs/src/schema/ directory with ~11 domain-concept pages
• Field tables, relationships, and usage examples per type
• Inline cross-links from all 16 existing API pages
• New "Schema" section in SUMMARY.md
• Only SDK types (not proto types, per Constitution I)

Context

Brainstorm document: brainstorm/023-schema-docs.md

Assisted-By: 🤖 Claude Code