activities: document participation field and --source filter
Participation shipped 4/18 (commit 101da52e) but skill was never updated.
Adds the schema (name, role ∈ {attendee, mentioned}, source ∈ {voice,
speaker_label, transcript, screen, other}, confidence, context) and a
worked example. Also adds the --source filter flag on list (anticipated,
user, cogitate).
Plus Invoke via Bash line and literal command names in TRIGGER.
Part of skills audit req_loq3e2lk pass 1+2.