fix: create the first persistent artifact on the incremental path

[mvanhorn]

What does this PR do?

Fixes #434: enabling persistence=true did not create the first persistent artifact on the incremental indexing path. dump_and_persist in pipeline_incremental.c only refreshed an artifact that already existed and never created one on first run, so a persistence=true request was silently a no-op until an artifact happened to exist. git ai stats-style consumers saw the index reported as successful with no persistent artifact written.

This threads a new cbm_pipeline_persistence() accessor into the incremental path (mirroring the existing cbm_pipeline_repo_path / cbm_pipeline_project_name accessors) so it exports the first artifact with CBM_ARTIFACT_BEST when persistence is requested and none exists, matching the full pipeline. Existing artifacts keep refreshing with CBM_ARTIFACT_FAST. It covers changed-file, deleted-file, and no-op incremental runs, and it now propagates a failed cbm_artifact_export as a run error instead of reporting a successful index when the artifact could not be written.

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)