refactor(system-lib): extract cj/shell-quote-argument-readable from dev-fkeys

Phase 2.2 of utility-consolidation. The "quote only when shell-unsafe characters appear, otherwise leave the argument readable" pattern was trapped in dev-fkeys as `cj/--f6-shell-quote-argument' alongside its `cj/--f6-shell-safe-argument-regexp' constant. Lift both into system-lib.el under their generic names; the F6 branding hid that the same shape is useful for any generated compile/test command where the surrounding line ends up in a *compilation* buffer the user reads.

Six Normal/Boundary tests cover safe inputs that pass through unchanged (alphanumeric paths, test regexes, `FLAG=value', `host:port'), unsafe inputs that get quoted (spaces, `$', `;', `&', backticks, `*'), and the empty-string boundary.

Migrate dev-fkeys's five callers to the new name and add `(require \='system-lib)' per the Phase 2 exit criterion.

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