refactor(external-open): consolidate OS-open dispatch in external-open.el

Phase 4 of utility-consolidation. Three previously-overlapping helpers (system-utils' `cj/identify-external-open-command' and `cj/--open-with-is-launcher-p', plus the dirvish-only `cj/--file-manager-program-for' shipped earlier today) all answered "which OS-open program should I run?".  Pull the answer into one place: external-open.el.

Move and rename:

- `cj/--open-with-is-launcher-p' (system-utils) -> `cj/external-open-launcher-p' (external-open). Public name now matches its module.
- `cj/identify-external-open-command' (system-utils) -> `cj/external-open-command' (external-open). Returns nil for unsupported hosts instead of signaling -- callers that need a command must handle nil explicitly. The wrapper `cj/xdg-open' (also moved into external-open) converts nil to a `user-error' with a clear message, preserving the user-facing failure shape.
- Delete dirvish's `cj/--file-manager-program-for' helper. `cj/dirvish-open-file-manager-here' now calls `cj/external-open-command' directly. The shell-command fallback for nil-program preserves the previous escape hatch.

Break the system-utils <-> external-open recursive require by moving `cj/xdg-open' (the only system-utils function that external-open used) into external-open along with the dispatch.

Tests reorganized to match the move. Two new test files (`test-external-open-command.el', `test-external-open-launcher-p.el') replace the two system-utils-named test files. The dirvish file-manager-program test goes away with the helper. 11 tests covering Normal/Boundary/Error for the dispatch (plus the new "unsupported host returns nil" contract).

Add `(require \='external-open)' to system-utils.el and `(require \='system-lib)' to external-open.el (for `cj/file-from-context' which xdg-open uses).

1 files changed, 2 insertions, 18 deletions

diff --git a/modules/dirvish-config.el b/modules/dirvish-config.el
index 89f6c397..d5834dac 100644
--- a/modules/dirvish-config.el
+++ b/modules/dirvish-config.el
@@ -246,21 +246,6 @@ Examples:
 
 ;;; ----------------------- Dirvish Open File Manager Here ----------------------
 
-(defun cj/--file-manager-program-for (has-xdg-open-p system-type)
-  "Return the file-manager command for HAS-XDG-OPEN-P + SYSTEM-TYPE, or nil.
-
-Pure helper used by `cj/dirvish-open-file-manager-here'.  When
-HAS-XDG-OPEN-P is non-nil, returns \"xdg-open\" regardless of
-SYSTEM-TYPE -- xdg-open works on Linux and many ported environments.
-Without xdg-open, falls back to `darwin' -> \"open\", `windows-nt' ->
-\"explorer\", everything else -> nil so the caller can shell-command
-its way out."
-  (cond
-   (has-xdg-open-p "xdg-open")
-   ((eq system-type 'darwin) "open")
-   ((eq system-type 'windows-nt) "explorer")
-   (t nil)))
-
 (defun cj/dirvish-open-file-manager-here ()
   "Open system's default file manager in the current dired/dirvish directory.
 Always opens the file manager in the directory currently being displayed,
@@ -271,9 +256,8 @@ regardless of what file or subdirectory the point is on."
         (progn
           (message "Opening file manager in %s..." current-dir)
           ;; Use pipe instead of pty for the async call-process below.
-          (let* ((process-connection-type nil)
-                 (program (cj/--file-manager-program-for
-                           (executable-find "xdg-open") system-type)))
+          (let ((process-connection-type nil)
+                (program (cj/external-open-command)))
             (if program
                 (call-process program nil 0 nil current-dir)
               (shell-command (format "xdg-open %s &"