fix: mark macOS clipboard copies as transient

[banteg]

Summary

• write macOS clipboard copies with org.nspasteboard.ConcealedType, org.nspasteboard.TransientType, and org.nspasteboard.AutoGeneratedType
• use printf instead of echo -n when piping copied password data
• avoid re-copying clipboard contents if the user changes the clipboard before the timeout expires
• restore the previous clipboard contents through the same marked pasteboard path when the password is still present
• fall back to pbcopy if osascript is unavailable

Motivation

Clipboard managers such as Raycast treat concealed/transient pasteboard items as sensitive and skip recording them in clipboard history. This matches the direction of #69 for Wayland wl-copy --sensitive, but applies to the Darwin platform helper.

The changed-timeout behavior also avoids creating a new clipboard-history event for content the user copied after passage --clip ran.

Testing

On macOS, sourced src/platform/darwin.sh and called clip with dummy values:

• verified the copied dummy value remains pasteable via pbpaste
• verified pasteboard types include org.nspasteboard.ConcealedType, org.nspasteboard.TransientType, and org.nspasteboard.AutoGeneratedType
• verified the timeout path restores the previous dummy clipboard value when the password is still present
• verified the timeout path leaves a user-changed dummy clipboard value untouched
• ran bash -n src/platform/darwin.sh