Summary

• Replaces the remaining namedSchemaError(...) calls with NamedError.create(...).
• Deletes packages/opencode/src/util/named-schema-error.ts.
• Consolidates shared session message errors in session/message-error.ts and reuses their schemas from v1/v2 message modules.

Verification

• bun typecheck in packages/core
• bun typecheck in packages/opencode
• bun run test -- test/util/error.test.ts test/session/retry.test.ts test/session/session-schema.test.ts test/provider/provider.test.ts
• bunx prettier --check ... on changed files
• bunx oxlint ... on changed files (warnings only, existing patterns)
• pre-push bun turbo typecheck