Add a `--stealth` flag to `bd prime` that outputs a simplified
workflow using only `bd sync --flush-only`, omitting all git
operations (commit, push, pull).
This addresses use cases where git operations need to be deferred
or handled separately from the bd workflow (e.g. bd init --stealth),
where committing files is may not desired as part of the Claude
conversation.
In stealth mode, the close protocol shows only the flush step.
Includes tests for current and existing functionality.
To make testing easier,
refactor output functions to accept `io.Writer` parameters
instead of writing directly to `os.Stdout`,
and convert `isEphemeralBranch` from a function to a
variable for stubbing.
Co-Authored-By: Claude <noreply@anthropic.com>
