refactor(protocol): harden simulation wire contract

[kitlangton]

What

Harden the public @opencode-ai/protocol/simulation contract before standalone Drive deletes its copied schema.

The canonical module now exposes typed backend notifications, models JSON-RPC responses as an exact success/error union, and preserves the existing protocol-v1 request, capability, and RPC declarations explicitly.

This advances #42136. After this lands in an OpenCode next release, Drive can pin that Protocol version and remove its 629-line copy.

How

• packages/protocol/src/simulation.ts
  • Models JSON-RPC responses with Schema.Union(..., { mode: "oneOf" }) and opposite Schema.Never fields, making success/error exclusivity hold at runtime and in TypeScript.
  • Exposes Backend.Notification and decoders for llm.request, tool.invocation, and tool.cancel.
  • Requires handshake capability responses to contain unique entries.
  • Keeps the explicit request unions and Effect RPC groups rather than introducing a generic operation framework.
• packages/simulation/test/protocol.test.ts
  • Covers typed notification decoding, finish-reason normalization, and JSON-RPC response exclusivity.
  • Includes compile-time checks for missing and conflicting response outcomes.
• packages/simulation/test/simulated-provider.test.ts
  • Exercises an omitted finish reason through the real websocket server.

Scope

This PR does not migrate opencode-drive; that follows the Protocol publication gate.

This PR does not change simulation activation. The typed launch profile and unified activation path remain separate follow-up work.

Testing

• bun typecheck in packages/protocol
• bun run build in packages/protocol
• bun typecheck in packages/simulation
• bun run test in packages/simulation (42 tests)
• bun run lint packages/protocol/src/simulation.ts packages/simulation/test/protocol.test.ts
• Repository-wide push typecheck across 33 packages

Flow

flowchart LR
  P[Protocol simulation contract] --> O[OpenCode simulation server]
  P -. published next version .-> D[Standalone Drive]
  D -. deletes copied schema .-> P

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