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.

1 files changed, 16 insertions, 0 deletions

diff --git a/modules/system-lib.el b/modules/system-lib.el
index 3ccec06b..96159179 100644
--- a/modules/system-lib.el
+++ b/modules/system-lib.el
@@ -80,6 +80,22 @@ Thin wrapper around `cj/process-output-or-error' with `git' as the
 program."
   (apply #'cj/process-output-or-error "git" args))
 
+(defun cj/file-from-context (&optional explicit-filename)
+  "Return a file path from the current context, or nil.
+
+Resolves in priority order:
+  1. EXPLICIT-FILENAME, if non-nil.
+  2. `buffer-file-name' of the current buffer.
+  3. The file at point if the current buffer is in `dired-mode'.
+
+Returns nil when none of these yield a file.  Useful for any command
+that operates on \"the current file\" -- buffer commands, dired
+commands, and external-open dispatchers all want this resolution."
+  (or explicit-filename
+      buffer-file-name
+      (and (derived-mode-p 'dired-mode)
+           (dired-file-name-at-point))))
+
 (defun cj/log-silently (format-string &rest args)
   "Append formatted message (FORMAT-STRING with ARGS) to *Messages* buffer.
 This does so without echoing in the minibuffer."