Post Task Artifact

Body for POST /tasks/{task_id}/artifacts (#2553).

An agent-posted, journal-anchored artifact. artifact_type selects the payload shape: report uses body (markdown); table uses columns and rows; link uses url and link_kind (preview / dashboard / document). poster is the claim identity: a caller may only post against a task whose claim it holds.

There is deliberately no progress field. Progress is a chain-computed projection of journaled work, never postable.

A posted artifact version plus its decoded content (for rendering).

content carries the type-specific fields (body for a report, columns/rows for a table, url/kind for a link). When the stored blob fails its hash check verified is False and content is omitted -- the surface must render tampered, never the bytes.