start: script failure with an empty error message logs nothing useful

[kitlangton]

Drive 2.0.0 (464f9a3).

When a script fails with an error whose message is empty, drive start prints ERROR and exits 1 with no cause, tag, or stack. Hit this with a ClientError from @opencode-ai/client wrapping a SchemaError (Expected Plugin.Info at ["data"][0]); the actual reason was only visible after adding Effect.tapCause(... Cause.pretty(cause)) to the script itself.

[2026-09-02T02:30:12.070Z] ERROR
error:
error: script "drive" exited with code 1

Expected: start renders the failure with Cause.pretty (or at least _tag plus the nested cause) so tagged errors with empty messages are diagnosable from the run log.

Log: .drive-output/plugin-registry/candidate-6.log; the cause is visible in candidate-8.log after the script-side workaround.

[kitlangton]

Reproduced with Drive 2.0.1 against OpenCode 0.0.0-beta-18955 (dea1f9cb7b). An Effect script calls opencode.session.wait after an accepted move from a source whose config contains a missing {file:./missing.txt} reference. OpenCode returns HTTP 500, but Drive prints only error: and writes an empty ERROR line to logs/opencode-drive.log. The OpenCode server log retains the ConfigInvalidError. Expected the script failure to preserve the operation and HTTP error/cause. This uses an isolated fixture, not a live project.