refactor(system-lib): extract cj/file-from-context from system-utils

Phase 2.4 of utility-consolidation, the last item in the spec's recommended order. `cj/--file-from-context' resolves "the current file" via a three-step fallback chain (explicit arg, `buffer-file-name', dired file at point) -- a useful pattern for any command that operates on the current file regardless of which kind of buffer the user is in.  Promote to public `cj/file-from-context' and re-home in system-lib.el so other modules (mail capture, external-open, AI conversation, dirvish helpers) can use it without an awkward dependency on system-utils.

Migrate the two callers in system-utils.el (`cj/open-this-file-with' and `cj/open-file-with-command') and add `(require \='system-lib)' there per the Phase 2 exit criterion.

Move the existing 7-test file to `tests/test-system-lib-file-from-context.el' and update its references to the new public name.  The test shape is unchanged: 4 Normal + 3 Boundary cases covering explicit-arg precedence, buffer-file-name fallback, dired fallback, and the all-nil case.

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