fix(dwim-shell): quote and validate user-controlled shell inputs

Several dwim-shell commands interpolated user-controlled strings straight into shell templates, so a value with spaces, quotes, or shell metacharacters could break out of the command. The worst was git-clone-clipboard-url, which dropped raw clipboard contents into "git clone <<cb>>".

I added three pure validators (git URL, ffmpeg timestamp, rename prefix) and fixed the interpolation sites. git-clone now validates the clipboard and passes the URL through shell-quote-argument instead of <<cb>>. The GPG recipient and the 7z archive name go through shell-quote-argument instead of hand-written single quotes. The thumbnail timestamp and the rename prefix are validated to a safe shape before they reach the command, so the unquoted interpolation that remains is constrained to digits, colons, and filename-safe characters.

The fifth case in the ticket, the video-concat filelist built with echo/tr/sed, is a redesign rather than a quoting fix and is filed as a follow-up.

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