Replaces plain <p> tags in BoardApprovalPayloadContent with MarkdownBody
(softBreaks enabled) so agent-authored markdown in these three fields —
headers, bullets, and newlines — renders correctly in the Board UI instead
of collapsing into a single unstyled paragraph. No schema change; the
fields remain plain strings in the approval payload, only the renderer
changed. Matches how comments, issue documents, and interaction cards
already render markdown via MarkdownBody.
Test coverage added for ## header → <h2>, bullet list → <ul><li>, and
plain-prose regression (no markup injected for single-line inputs).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>