SMOODEV-tsdown: drop unused tsup root devDependency

[brentrager]

Summary

The observability monorepo already builds with tsdown (see `packages/core/tsdown.config.ts`); the root `tsup` devDependency was a leftover from before the migration and is unreferenced by any build script. Dropping it cleans up the install graph.

Test plan

• `pnpm install` clean
• `pnpm build` clean
• CI green

🤖 Generated with Claude Code

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Latest commit: b9a4d5a

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