fix(org): guard external-tool assumptions in export and publishing commands

Four export/publishing commands shelled out to external tools without checking they exist, so a missing tool surfaced as an opaque process error — or, for reveal.js, a silently broken presentation. I added a command-time guard to each that names the tool and what's needed:

- zathura, in the pandoc PDF export-and-open command
- the hugo binary and the platform file-manager opener, in hugo-config
- the local reveal.js checkout (run scripts/setup-reveal.sh), shared by the reveal export and preview commands
- pandoc, in the web-clip protocol handler

The checks run only when the command runs, so startup stays quiet. Each guard has a test asserting the user-error fires when the tool is absent, and the existing happy-path tests now stub the lookups so they exercise the real path rather than tripping the new guard.

1 files changed, 3 insertions, 0 deletions

diff --git a/modules/org-webclipper.el b/modules/org-webclipper.el
index 9c2f10617..99e837e63 100644
--- a/modules/org-webclipper.el
+++ b/modules/org-webclipper.el
@@ -168,6 +168,9 @@ It fetches the page content and converts it to Org format."
   ;; and fail with a `void-variable widget-field-keymap' error from
   ;; the customize machinery loading lazily.
   (require 'org-web-tools)
+  (unless (executable-find "pandoc")
+    (user-error
+     "pandoc not found on PATH; it is required to convert the clipped page to Org"))
   (setq org-web-tools-pandoc-sleep-time 0.5)
 
   (let ((url cj/--webclip-url)