Why: T2 of the interaction foundation — activity cards as first-class state
objects behind the right-rail inbox (proposal D25): upload ingest cards
(SR-A5 failures explicit, artifact retrievable), HITL projection cards
(double-presence, ref-linked two-way sync), squad orchestration cards and
the mock #upload demo of the full card lifecycle.
Evidence: go test ./... green across 14 packages incl. new cardsvc suite
(lifecycle, action labels and idempotency, pin/dismiss, owner isolation,
resolve-by-ref idempotency) and updated migration test (v6 activity_cards,
both dialects); events flow through the session event buffer (persist, seq,
replay); receipts appended to the conversation timeline on card actions.
Impact: additive; cards disabled (SetCards nil) degenerates to the T0
baseline; upload cards persist at project level per the v1 inbox boundary
(conversation-scoped inbox; project-level aggregation is follow-up).
Refs: modules/documents docs/proposals/2026-09-30-ui-inbox-command-workspace.md
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