fix: replace clipboardy with native clipboard commands (#162)

[davideast]

Summary

• Removes clipboardy (and @types/clipboardy) from dependencies. It bundles clipboard_x86_64.exe and clipboard_i686.exe as Windows fallbacks, which enterprise Threat Intelligence systems flag as malicious binaries and block stitch-mcp installation in managed environments (clipboard_x86_64.exe in clipboardy fallbacks flagged as Malicious Binary by Threat Intelligence #162).
• Replaces it with a small in-house module that pipes text to platform-native clipboard commands: pbcopy on darwin, clip.exe on win32, and wl-copy/xclip/xsel (in that order) on linux.
• Exported API (copyText, copyJson, downloadAndCopyText, downloadAndCopyImage) is unchanged; callers in checklist.ts, AuthStep.ts, and ScreensView.tsx swap imports without further changes.

Fixes #162.

Test plan

• bun test — 485 pass, 6 skip, 0 fail (no new skips)
• npx tsc --noEmit — zero new errors in touched files (only pre-existing test-file errors)
• bun scripts/verify-pack.ts — clean
• npm pack tarball contains zero .exe files
• darwin smoke test: copyText → pbpaste round-trip works
• Windows smoke test (manual, post-merge): clip.exe round-trip
• Linux smoke test (manual, post-merge): xclip / wl-copy round-trip