v0.2.5 — no server text leaks into chat

Fixed

• No OpenCode server text leaks into Talk. The prompt worker caught generic exceptions and posted Fehler: {exc}; an OpenCodeError carries up to ~300 chars of the OpenCode HTTP response body, which could reach the chat. The worker (session-setup and prompt paths) now catches OpenCodeError and posts a generic oc_error message, logging the details only — consistent with the command handlers. Remaining error: {exc} paths only fire for local validation errors (e.g. a bad model string), which carry no server body.

Regression tests assert the server body never reaches chat.

Full diff: v0.2.4...v0.2.5