refactor(system-utils): extract testable open-file helpers

Extracts two pure helpers from cj/open-file-with-command and cj/xdg-open so the file-resolution and launcher-detection logic becomes testable without mocking process launchers.

New helpers:

- cj/--file-from-context returns a file path from the current context, resolving in priority order (explicit arg, buffer-file-name, dired file at point). Returns nil when none apply.
- cj/--open-with-is-launcher-p is a predicate for whether a command is a desktop launcher (xdg-open, open, start) that needs call-process detachment.

Both commands now delegate. cj/open-file-with-command uses cj/--file-from-context with read-file-name as the final fallback, plus cj/--open-with-is-launcher-p for the launcher dispatch. cj/xdg-open uses cj/--file-from-context with user-error as the "no file" fallback.

Behavior preserved. The existing system-utils test suites still pass, and the shape of each command's final effect is identical.

New tests, 14 cases across two per-function files:

- tests/test-system-utils--file-from-context.el covers: explicit wins over buffer-file, explicit wins over dired, buffer-file fallback, dired fallback, all-nil returns nil, explicit-nil uses chain, dired-mode-but-no-file-at-point.
- tests/test-system-utils--open-with-is-launcher-p.el covers: each of the three launcher names returns t, non-launcher returns nil, empty string returns nil, case-sensitive check, nil input returns nil.

Coverage: system-utils.el went from 10/52 (19.2%) to 15/52 (28.8%). The remaining uncovered lines are mostly in the process-launching paths of cj/open-file-with-command and cj/xdg-open. Those are testability-blocked. Mocking call-process, start-process-shell-command, and generate-new-buffer would give a lot of mock surface for low value. cj/server-shutdown is not meaningfully testable because it kills Emacs.

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