chore: migrate build tool from tsup to tsdown

[amotarao]

Summary

• tsup から tsdown へビルドツールを移行
• tsdown.config.ts を新規追加、tsup.config.ts を削除
• package.json の exports を .mjs / .d.mts に更新（tsdown の ESM デフォルト出力に合わせて）

検証

• tsup / tsdown の出力を比較：エクスポート・型シグネチャは完全一致（インデント・import 順・region コメントのみ差異）
• tsdown は TypeScript 6.x の baseUrl 非推奨エラーが出ない（tsup は ignoreDeprecations: "6.0" が必要だった問題が解消）
• build & test パス確認済み

Test plan

• pnpm --filter chat-adapter-backlog build が成功すること
• pnpm --filter chat-adapter-backlog test が成功すること

Closes PC-50

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 50f20774-5efd-4b1b-b4d1-cc4d9ecf1522

📥 Commits

Reviewing files that changed from the base of the PR and between 887fdeb and e29df45.

⛔ Files ignored due to path filters (1)

• pnpm-lock.yaml is excluded by !**/pnpm-lock.yaml

📒 Files selected for processing (3)

• .changeset/tsdown-migration.md
• packages/chat-adapter-backlog/package.json
• packages/chat-adapter-backlog/tsdown.config.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch amon/pc-50-tsdown

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