Summary

• add one shared, non-empty renderer for unknown errors and Effect Causes
• use it at BackgroundJob, integration status, and session tool-failure boundaries instead of squashing to .message
• give provider, model-resolution, process, tool-output, decode, filesystem, and system-context errors meaningful derived messages
• reuse the core renderer from the TUI while keeping diagnostic stack formatting separate

Why

Schema.TaggedErrorClass does not derive .message from structured fields. Errors such as ProviderModelNotFoundError therefore reached BackgroundJob with an empty message, producing Error for foreground tasks and an empty <task_error> for background tasks.

Verification

• bun typecheck in packages/core
• bun typecheck in packages/tui
• bun typecheck in packages/opencode
• 62 focused core tests across error rendering, BackgroundJob, process, model resolution, and tool output
• 4 TUI error utility tests
• 88 provider tests
• oxlint on changed files (no new warnings)
• git diff --check