feat(blocks): add video, plan, and task_card block support

[zhawtof]

Summary

Adds builder support for the three remaining Slack block types the validator already understands but the palette and editor surface didn't expose: video, plan, and task_card.

Why

The validator schema already accepts video, plan, and task_card, and slack-blocks-to-jsx already renders them, but block-kitchen had no SupportedBlockType entry, palette variant, or editor for any of them. That gap meant users couldn't compose the standard Slack media block (video) or the newer agent-facing blocks (plan, task_card) from the builder — they had to hand-roll the payloads.

This PR closes the gap so the builder covers Slack's full app-sendable surface (everything except file, which Slack only emits when reading messages and apps cannot send outbound).

What changed

• Types (src/types.ts): new VideoBlock, PlanBlock, TaskCardBlock, TaskCardStatus, and UrlSourceElement interfaces; expanded SupportedBlockType and SupportedBlock unions.
• Palette (src/lib/default-blocks.ts):
  • Agents section gets Plan, Task card, and Task card with sources variants.
  • New Video section with Basic and With metadata variants.
  • labelForBlockType gets cases for the three new types.
• Editors: new video-editor.tsx, plan-editor.tsx, task-card-editor.tsx. Plan and task_card use a RadioGroup for status; task_card includes a sources sub-editor (add/remove rows of label + URL).
• Wiring: block-editor.tsx dispatch cases, block-row.tsx label map, and src/index.ts public type exports.

The rich-text details and output fields on task_card round-trip on the payload but aren't editable inline in v1 — palette variants that include them keep them on save. A TaskCardEditor help line calls this out.

Test plan

• pnpm typecheck
• pnpm lint
• pnpm test — 217/217 pass, including the every default palette factory produces a payload that validates check, which exercises each new factory through toSlackBlocks → validateBlockKit.
• Manually verified in pnpm demo:dev:
  • All 5 new palette variants drop into the preview surface.
  • All 3 new editors open with seeded values populated (Plan title + 3 tasks with status radios; Video's 9 fields; Task card title/id/sources/status).
  • The preview renders the blocks via slack-blocks-to-jsx.

Notes for reviewer

slack-blocks-to-jsx's video renderer puts a <p> containing a <div>, which React flags as a hydration warning in the console. The warning originates inside the library's own vl component (not our payload) and the block renders correctly; not addressed here.

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	block-kitchen	e26c057	Commit Preview URL Branch Preview URL	May 20 2026, 04:41 AM