fix: include offending branch name in trigger.pipeline validation error message

[jamesadevine]

Problem

When a branch entry in triggers.pipeline.branches contains a newline, the error message does not include which branch triggered the failure. This makes debugging harder for users with multiple branches configured.

Suggested Fix

Include the offending value in the error:

anyhow::bail!(
    "Front matter 'triggers.pipeline.branches' entry {:?} must be single line (no newlines). \
     Multi-line values could inject YAML structure into the generated pipeline.",
    branch,
);

Context

Found during review of #184.