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, 2 insertions, 0 deletions

diff --git a/modules/org-export-config.el b/modules/org-export-config.el
index 493316f9..5a6f09fc 100644
--- a/modules/org-export-config.el
+++ b/modules/org-export-config.el
@@ -98,6 +98,8 @@
   (defun my/org-pandoc-export-to-pdf-and-open ()
 	"Export to PDF via pandoc and open with Zathura."
 	(interactive)
+	(unless (executable-find "zathura")
+	  (user-error "Cannot open the exported PDF: zathura is not installed or not on PATH"))
 	(let ((pdf-file (org-pandoc-export-to-latex-pdf)))
 	  (when pdf-file
 		(start-process "zathura-pdf" nil "zathura" pdf-file)