fix: sanitize calendar event headings and property values

`calendar-sync--event-to-org` already cleaned the description body via `calendar-sync--sanitize-org-body`, but the event summary went into the heading line and the location, organizer, status, and URL went into the property drawer without sanitization. Any of those fields containing newlines could create extra Org headings, close the property drawer early with a stray `:END:`, or inject property-looking lines that the agenda would then parse as real properties.

I added two helpers. `calendar-sync--sanitize-org-property-value` trims the input and collapses any run of whitespace or newlines into a single space. `calendar-sync--sanitize-org-heading` composes that over the existing body sanitizer so `*` sequences also become `-`. The event-to-org function now routes the summary through the heading sanitizer and each property value through the property sanitizer.

I added regression tests across two files. `test-calendar-sync--sanitize-org-body.el` gets 4 new tests for the two helpers, covering newline flattening, leading-star replacement, structural-character flattening, and whitespace collapse. `test-calendar-sync--event-to-org.el` gets 2 new integration tests. A summary containing `\n** Hidden task` produces a single `* ` heading with the body inlined. A location containing `\n:END:\n* Not a real heading` collapses to a single property line with no extra `:END:` or heading injected.

515 calendar-sync tests pass together.

1 files changed, 28 insertions, 5 deletions

diff --git a/modules/calendar-sync.el b/modules/calendar-sync.el
index b9cc57f4..06bee213 100644
--- a/modules/calendar-sync.el
+++ b/modules/calendar-sync.el
@@ -287,6 +287,20 @@ parsed as org headings.  Handles multiple levels (e.g. ** becomes --)."
        (concat (make-string (length (match-string 1 match)) ?-) " "))
      text)))
 
+(defun calendar-sync--sanitize-org-property-value (text)
+  "Sanitize TEXT for safe inclusion as a single Org property value."
+  (when text
+    (string-trim
+     (replace-regexp-in-string
+      "[[:space:]\n\r]+"
+      " "
+      text))))
+
+(defun calendar-sync--sanitize-org-heading (text)
+  "Sanitize TEXT for safe inclusion as a single Org heading title."
+  (calendar-sync--sanitize-org-property-value
+   (calendar-sync--sanitize-org-body text)))
+
 ;;; Date Utilities
 
 (defun calendar-sync--add-months (date months)
@@ -1059,7 +1073,8 @@ Cleans text fields (description, location, summary) via `calendar-sync--clean-te
   "Convert parsed EVENT plist to org entry string.
 Produces property drawer with LOCATION, ORGANIZER, STATUS, URL when present.
 Description appears as body text after the drawer."
-  (let* ((summary (or (plist-get event :summary) "(No Title)"))
+  (let* ((summary (calendar-sync--sanitize-org-heading
+                   (or (plist-get event :summary) "(No Title)")))
          (description (plist-get event :description))
          (location (plist-get event :location))
          (start (plist-get event :start))
@@ -1072,16 +1087,24 @@ Description appears as body text after the drawer."
          (props '()))
     ;; Collect non-nil properties
     (when (and location (not (string-empty-p location)))
-      (push (format ":LOCATION: %s" location) props))
+      (push (format ":LOCATION: %s"
+                    (calendar-sync--sanitize-org-property-value location))
+            props))
     (when organizer
       (let ((org-name (or (plist-get organizer :cn)
                           (plist-get organizer :email))))
         (when org-name
-          (push (format ":ORGANIZER: %s" org-name) props))))
+          (push (format ":ORGANIZER: %s"
+                        (calendar-sync--sanitize-org-property-value org-name))
+                props))))
     (when (and status (not (string-empty-p status)))
-      (push (format ":STATUS: %s" status) props))
+      (push (format ":STATUS: %s"
+                    (calendar-sync--sanitize-org-property-value status))
+            props))
     (when (and url (not (string-empty-p url)))
-      (push (format ":URL: %s" url) props))
+      (push (format ":URL: %s"
+                    (calendar-sync--sanitize-org-property-value url))
+            props))
     (setq props (nreverse props))
     ;; Build output
     (let ((parts (list timestamp (format "* %s" summary))))