fix: make package builds cross-platform

[Hushlor]

Summary

• replace Unix-only cp and chmod package build steps with a dependency-free Node dist helper
• preserve server runtime asset copying and POSIX executable permissions while safely skipping mode changes on Windows
• verify both server and CLI builds across Windows, macOS, and Linux in CI
• add regression tests for asset filtering, missing inputs, entry-point validation, and platform-specific permissions

Verification

• npm run build (full workspace, native Windows)
• npm run build -w @hypothesi/tauri-mcp-server
• npm run build -w @hypothesi/tauri-mcp-cli
• npm run test:unit -w @hypothesi/tauri-mcp-server (65 passed, 1 POSIX-only test skipped on Windows)
• npm test -w @hypothesi/tauri-mcp-cli (2 passed with live test app)
• npm run standards
• npm pack --dry-run --json for server and CLI
• workflow YAML parse validation

The server package dry-run contains all 11 source runtime JavaScript assets, no raw TypeScript sources, and the expected entry point. The CLI package contains both expected bin entry points. Ubuntu/macOS CI executes the POSIX mode assertion that is intentionally skipped on Windows.

Closes #43