Show underlying causes for triage agent failures

[matthewp]

Changes

• Preserve Flue settlement cause chains before Cloudflare Workflow step serialization drops Error.cause.
• Include the pipeline stage and submission ID in triage failure reports, making infrastructure and timeout failures actionable from the issue comment.

Testing

• Adds coverage for serialized settlement causes and non-Error failures.

Docs

• No docs update needed because this only improves internal failure diagnostics.