SlipJay errors keep their cause and carry no raw CLI output #51

Part of #21. From the final Spec review of #21 (Standards 1).

Outcome

SlipJay failures follow docs/internals/effect-services.md §Errors: no raw CLI output in error details, and the immediate cause kept.

Problem found

In apps/server/src/project/SlipJayLandings.ts, readLog puts output.stderr straight into GitCommandError.detail, and the web landing log dialog shows it. runLand maps a spawn failure to a fixed message and drops the cause. Both are typed as GitCommandError although they are SlipJay operations.

Acceptance

• Details name the failed operation without raw stderr or command output; the underlying error is kept as cause.
• Behaviour tests cover a failing slipjay runner log and a slipjay land that cannot be spawned.
• The landing log dialog still tells the user why a log could not be read, in the product's words.