Remove Zod from named errors

[kitlangton]

Summary

• Moves NamedError from Zod-backed schemas to Effect Schema while preserving the legacy { name, data } wire shape.
• Converts direct NamedError.create(...) call sites to Effect Schema fields and makes namedSchemaError return real NamedError instances.
• Keeps config issue metadata and void-error JSON compatibility intact.

Verification

• bun typecheck in packages/core
• bun typecheck in packages/opencode
• bun run test -- test/util/error.test.ts test/cli/error.test.ts test/server/sdk-error-shape.test.ts test/server/httpapi-schema-error-body.test.ts test/session/retry.test.ts test/config/config.test.ts test/skill/skill.test.ts test/storage/storage.test.ts test/project/worktree.test.ts
• bunx prettier --check ... on changed files
• bunx oxlint ... on changed files (warnings only, existing patterns)
• pre-push bun turbo typecheck