fix: add @lydell/node-pty to devDependencies for build process

[blinkagent]

Problem

CI was failing at the build step with the error:

ERROR Error: Cannot find module '@lydell/node-pty'

The build process in tsdown.config.ts uses import.meta.resolve("@lydell/node-pty") to resolve the package path, but the package was not listed in the devDependencies.

Additionally, tsdown was not available to all workspace packages, causing builds to fail for packages other than blink.

Solution

1. Added @lydell/node-pty (^1.1.0) to the devDependencies in packages/blink/package.json.
2. Added tsdown (0.14.2) to the root package.json devDependencies so all workspace packages can access it.

The package is used during the build process to:

1. Download platform-specific native binaries
2. Bundle them into the CLI distribution
3. Replace import paths for the bundled binaries

This is needed for the CLI to support TTY operations on different platforms.

Status

✅ Build step now passes - All packages build successfully

⚠️ Typecheck failures are pre-existing - The typecheck step fails with missing peer dependencies (@blink-sdk/multiplexer, file-type, ai, @octokit/types, @slack/types, etc.). These errors existed before this PR and are unrelated to the build fixes.

This PR fixes the original CI build failure. The typecheck issues should be addressed in a separate PR.