refactor(system-lib): extract cj/process-output-or-error and cj/git-output-or-error from coverage-core

Phase 2.3 of utility-consolidation. `cj/--coverage-git-string' was a generic argv-based runner ("run program, return stdout, raise user-error on non-zero with status+output in the message") trapped inside coverage-core.  Lift the generic shape into `cj/process-output-or-error' and add `cj/git-output-or-error' as a one-line wrapper that supplies "git" as the program.  Both live in system-lib.el.

Future callers I have in mind: reconcile-open-repos shell-style git calls (the high-priority data-safety task), vc-config clipboard cloning, mail integrations that touch git for commit signatures.

Six Normal/Boundary/Error tests cover success/no-args/non-zero-exit for the generic runner, the user-error message content (program name, exit status, trimmed output), and the git wrapper's program argument routing.

Migrate coverage-core's `cj/--coverage-git-merge-base' and `cj/--coverage-git-diff' to call the new git wrapper.  Drop the local `cj/--coverage-git-string' definition.  Add `(require \='system-lib)' to coverage-core.el per the Phase 2 exit criterion.

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