Remove effect-zod bridge

[kitlangton]

Summary

• delete the generic @opencode-ai/core/effect-zod Schema-to-Zod bridge and its tests
• add a native Effect Schema tool JSON Schema helper used by prompt/tool-listing paths
• keep plugin Zod compatibility isolated at the tool registry boundary via a jsonSchema side channel
• update Effect migration docs to prefer native Effect/JSON/Standard Schema boundaries

Verification

• bun typecheck in packages/opencode
• bun typecheck in packages/core
• bun run test -- test/tool/parameters.test.ts test/tool/registry.test.ts test/session/retry.test.ts
• pre-push bun turbo typecheck