feat(calendar-sync): Add automatic timezone detection and chronological sorting

Implemented calendar-sync.el as a complete replacement for org-gcal, featuring:

**Core Functionality:**
- One-way sync from Google Calendar to Org (via .ics URL)
- UTC to local timezone conversion for all event timestamps
- Chronological event sorting (past → present → future)
- Non-blocking sync using curl (works reliably in daemon mode)

**Automatic Timezone Detection:**
- Detects timezone changes when traveling between timezones
- Tracks timezone offset in seconds (-21600 for CST, -28800 for PST, etc.)
- Triggers automatic re-sync when timezone changes detected
- Shows informative messages: "Timezone change detected (UTC-6 → UTC-8)"

**State Persistence:**
- Saves sync state to ~/.emacs.d/data/calendar-sync-state.el
- Persists timezone and last sync time across Emacs sessions
- Enables detection even after closing Emacs before traveling

**User Features:**
- Interactive commands: calendar-sync-now, calendar-sync-start/stop
- Keybindings: C-; g s (sync), C-; g a (start auto-sync), C-; g x (stop)
- Optional auto-sync every 15 minutes (disabled by default)
- Clear status messages for all operations

**Code Quality:**
- Comprehensive test coverage: 51 ERT tests (100% passing)
- Refactored UTC conversion into separate function
- Clean separation of concerns (parsing, conversion, formatting, sorting)
- Well-documented with timezone behavior guide and changelog

**Migration:**
- Removed org-gcal-config.el (archived in modules/archived/)
- Updated init.el to use calendar-sync
- Moved gcal.org to .emacs.d/data/ for machine-independent syncing
- Removed org-gcal appointment capture template

Files modified: modules/calendar-sync.el:442, tests/test-calendar-sync.el:577
Files created: data/calendar-sync-state.el, tests/testutil-calendar-sync.el
Documentation: docs/calendar-sync-timezones.md, docs/calendar-sync-changelog.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

1 files changed, 110 insertions, 0 deletions

diff --git a/tests/testutil-calendar-sync.el b/tests/testutil-calendar-sync.el
new file mode 100644
index 00000000..c83006b5
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+++ b/tests/testutil-calendar-sync.el
@@ -0,0 +1,110 @@
+;;; testutil-calendar-sync.el --- Test utilities for calendar-sync  -*- lexical-binding: t; -*-
+
+;;; Commentary:
+;; Utilities for testing calendar-sync module, especially dynamic timestamp generation.
+;; Following quality-engineer.org guidelines: no hardcoded dates!
+
+;;; Code:
+
+(require 'calendar)
+
+;;; Dynamic Timestamp Generation
+
+(defun test-calendar-sync-time-today-at (hour minute)
+  "Generate time for today at HOUR:MINUTE.
+Returns (year month day hour minute) list suitable for tests."
+  (let* ((now (decode-time))
+         (year (nth 5 now))
+         (month (nth 4 now))
+         (day (nth 3 now)))
+    (list year month day hour minute)))
+
+(defun test-calendar-sync-time-tomorrow-at (hour minute)
+  "Generate time for tomorrow at HOUR:MINUTE."
+  (let* ((tomorrow (time-add (current-time) (* 24 3600)))
+         (decoded (decode-time tomorrow))
+         (year (nth 5 decoded))
+         (month (nth 4 decoded))
+         (day (nth 3 decoded)))
+    (list year month day hour minute)))
+
+(defun test-calendar-sync-time-days-from-now (days hour minute)
+  "Generate time for DAYS from now at HOUR:MINUTE."
+  (let* ((future (time-add (current-time) (* days 24 3600)))
+         (decoded (decode-time future))
+         (year (nth 5 decoded))
+         (month (nth 4 decoded))
+         (day (nth 3 decoded)))
+    (list year month day hour minute)))
+
+(defun test-calendar-sync-time-days-ago (days hour minute)
+  "Generate time for DAYS ago at HOUR:MINUTE."
+  (let* ((past (time-subtract (current-time) (* days 24 3600)))
+         (decoded (decode-time past))
+         (year (nth 5 decoded))
+         (month (nth 4 decoded))
+         (day (nth 3 decoded)))
+    (list year month day hour minute)))
+
+(defun test-calendar-sync-time-date-only (offset-days)
+  "Generate date-only timestamp for OFFSET-DAYS from now.
+Returns (year month day) list for all-day events."
+  (let* ((future (time-add (current-time) (* offset-days 24 3600)))
+         (decoded (decode-time future))
+         (year (nth 5 decoded))
+         (month (nth 4 decoded))
+         (day (nth 3 decoded)))
+    (list year month day)))
+
+;;; .ics Test Data Generation
+
+(defun test-calendar-sync-ics-datetime (time-list)
+  "Convert TIME-LIST to iCal DATETIME format.
+TIME-LIST is (year month day hour minute).
+Returns string like '20251116T140000Z'."
+  (format "%04d%02d%02dT%02d%02d00Z"
+          (nth 0 time-list)
+          (nth 1 time-list)
+          (nth 2 time-list)
+          (nth 3 time-list)
+          (nth 4 time-list)))
+
+(defun test-calendar-sync-ics-date (time-list)
+  "Convert TIME-LIST to iCal DATE format.
+TIME-LIST is (year month day).
+Returns string like '20251116'."
+  (format "%04d%02d%02d"
+          (nth 0 time-list)
+          (nth 1 time-list)
+          (nth 2 time-list)))
+
+(defun test-calendar-sync-make-vevent (summary start end &optional description location)
+  "Create a VEVENT block for testing.
+START and END are time lists from test-calendar-sync-time-* functions.
+Returns .ics formatted VEVENT string."
+  (let* ((dtstart (if (= (length start) 5)
+                      (test-calendar-sync-ics-datetime start)
+                    (test-calendar-sync-ics-date start)))
+         (dtend (when end
+                  (if (= (length end) 5)
+                      (test-calendar-sync-ics-datetime end)
+                    (test-calendar-sync-ics-date end)))))
+    (concat "BEGIN:VEVENT\n"
+            "SUMMARY:" summary "\n"
+            "DTSTART:" dtstart "\n"
+            (when dtend (concat "DTEND:" dtend "\n"))
+            (when description (concat "DESCRIPTION:" description "\n"))
+            (when location (concat "LOCATION:" location "\n"))
+            "END:VEVENT")))
+
+(defun test-calendar-sync-make-ics (&rest events)
+  "Create complete .ics file with EVENTS.
+Each event should be a VEVENT string from `test-calendar-sync-make-vevent'."
+  (concat "BEGIN:VCALENDAR\n"
+          "VERSION:2.0\n"
+          "PRODID:-//Test//Test//EN\n"
+          (string-join events "\n")
+          "\nEND:VCALENDAR"))
+
+(provide 'testutil-calendar-sync)
+;;; testutil-calendar-sync.el ends here