Rename block-kit-builder to block-kitchen

[zhawtof]

Summary

• Renames the underlying library dependency @tightknitai/block-kit-builder → @tightknitai/block-kitchen and updates the React import to BlockKitchen / BlockKitchenProps.
• Renames this template (package name, Worker name, launch config, Slack app display name, HTML title, h1) and updates the README, SECURITY, and issue-template URLs and labels.
• Leaves Slack's own "Block Kit" product terminology intact: the @tightknitai/slack-block-kit-validator dependency, the "Block Kit message" fallback text in the Worker, and "Block Kit validation" / "your first Block Kit message" prose.

Why

The underlying React component library is being rebranded from block-kit-builder to block-kitchen; the template needs to follow the rename so it depends on the published package under its new name and presents the new brand to users.

Reviewer notes

• pnpm-lock.yaml was deliberately not touched — it still pins @tightknitai/block-kit-builder@0.1.0-alpha.0. Run pnpm install to regenerate against the new package name (requires @tightknitai/block-kitchen to be published).
• The renamed React export (BlockKitchen, BlockKitchenProps) assumes the library's TypeScript exports are being renamed alongside the package. If the library kept the old export names, revert the identifier changes in src/client/App.tsx.
• The on-disk directory is still block-kit-builder-template — rename the repo/directory separately if desired.

Test plan

• Publish @tightknitai/block-kitchen (or update version) so pnpm install resolves.
• pnpm install regenerates pnpm-lock.yaml cleanly.
• pnpm run typecheck passes (confirms BlockKitchen / BlockKitchenProps exports exist).
• pnpm run build succeeds.
• pnpm run dev:tunnel — load /, confirm the h1 reads "Block Kitchen Template" and the builder mounts.
• Drag a block, click Send, confirm a message posts to Slack (verifies the renamed component still wires loadChannels / loadSendAsUserStatus / onSend correctly).

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