feat(signals): pipeline audience targets for steering notes

What Let users address a steering note to the report pipeline specifically, distinct from one scout or the whole fleet, via a reserved audience family on the note target: pipeline:report-research now, with room for pipeline:implementation later. Why With the research agent reading notes (#91267), the only note targets are one scout skill (skill_name) or everything (blank). Users need "tell the report generator X" without also steering every scout — e.g. "route billing-adjacent reports to the billing folks" is research-stage guidance, not fleet guidance. Naming matters here: the target must be family-shaped (pipeline:*) rather than a bare name like report-research that reads as a scout skill which does not exist. Implementation sketch • products/signals/backend/scout_harness/tools/notes.py: leave_note validates a targeted skill_name; carve out reserved pipeline:* audiences so they skip the scout-skill lookup. list_notes for a pipeline audience returns pi…