feat: add JSON Schema as source of truth for type definitions

[TonyMarkham]

Summary

This PR implements JSON Schema as the canonical source of truth for OpenCode's type definitions, addressing #6879.

72+ JSON Schema files now define all major type domains, with Zod validators auto-generated from them.

What's Included

Domain	Schemas	Key Types
Model	8 files	ModelInfo, ModelCost, ModelCapabilities, ModelLimits
Provider	6 files	ProviderInfo, ProviderOptions, ProviderSource
Auth	4 files	Auth, ApiAuth, OAuth, WellKnownAuth
Session	10 files	SessionInfo, SessionTime, SessionSummary, FileDiff
Message	20 files	Message, UserMessage, AssistantMessage, all Part types, Error types
Tool	9 files	ToolState, ToolPart, PermissionRequest, PermissionReply
Agent	2 files	AgentInfo, AgentModel
Event	13 files	Event, GlobalEvent, SessionStatus, all event types

Implementation

1. Custom code generator (script/generate-from-schemas.ts)

   • Validates schemas with AJV
   • Generates Zod validators + TypeScript types
   • Handles cross-file $ref correctly (existing tools don't)
   • Integrated with bun run generate workflow

2. Refactored source files to use generated validators:

   • provider.ts, message-v2.ts, permission/next.ts
   • agent/agent.ts, session/status.ts, server/server.ts

3. Documentation in schema/README.md

Results

• ✅ All 87 schemas validate
• ✅ All 544 tests pass
• ✅ TypeScript compilation passes
• ✅ Build succeeds (11 platforms)
• ✅ Runtime behavior unchanged

Benefits for External Clients

# Rust
typify schema/modelInfo.schema.json

# Python
datamodel-codegen --input schema/ --output opencode_types/

# C#
NJsonSchema.CodeGeneration schema/modelInfo.schema.json

Technical Notes

• Generated files are gitignored (build artifacts)
• Zod 4.x compatible
• z.union() → z.discriminatedUnion() where schema has discriminator (behavioral equivalence)

Closes #6879