build: upgrade to TypeScript 6 and tsdown

[bytaesu]

docs already need updates and aren't currently deployed automatically, so I left them unchanged in this PR

[Copilot]

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[greptile-apps]

Greptile Summary

This PR migrates both better-fetch and logger packages from tsup to tsdown (rolldown-based) and bumps TypeScript from ^5.x to ^6.0.3. A small explicit return-type annotation is added to getTimeout in utils.ts to satisfy TypeScript 6's stricter inference requirements.

• Build tool migration: tsdown.config.ts files replace tsup.config.ts across both packages, mapping external → deps.neverBundle and adding target: "es2017" / platform: "neutral". Default output extensions (.js for ESM, .cjs for CJS) align with the exports fields in each package.json.
• TypeScript 6 compatibility: tsconfig.json files updated with lib: ["ES2022", "DOM"] and moduleResolution: "bundler" where needed; packages/logger/tsconfig.json switches module from "commonjs" to "ESNext" and adds customConditions: ["node"].
• Toolchain centralization: tsdown is declared only in the root package.json rather than in each package's own devDependencies (unlike the previous tsup setup), relying on pnpm workspace hoisting for resolution.

Confidence Score: 5/5

Safe to merge; the tooling changes are mechanical and the output file paths produced by tsdown's defaults correctly match the exports fields in both packages.

The build configs produce the correct ESM/CJS output extensions without any outExtensions override, the tsconfig changes are consistent with TypeScript 6 requirements, and the explicit return-type annotation in utils.ts is accurate. No logic changes to library code are introduced.

packages/better-fetch/package.json and packages/logger/package.json no longer declare tsdown locally (unlike the previous tsup entries); this relies on pnpm workspace hoisting and is worth keeping an eye on if the packages are ever built in an isolated context.

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