fix: old links never re-create ended sessions

Inbox entries, the switcher, palette switches, the attention chip and Matrix
links (?arg=NAME&existing=1) attach with --existing; an ended session gets an
explanation and an explicit "Start again". Inbox entries for ended sessions
can't resurrect them; entries expire after 7 days.

Closes #38

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libexec/devdesk-notify-matrix

@@ -28,7 +28,9 @@ attached=$(dd_tmux list-sessions -F '#{session_name} #{session_attached}' 2>/dev
 
 message=$(jq -rM '.message // "Claude is waiting for your input"' "$state_file" 2>/dev/null)
 url=""
-[ -z "$DEVDESK_PUBLIC_URL" ] || url="${DEVDESK_PUBLIC_URL%/}/?arg=$session"
+# existing=1: if the session was ended by the time you tap, say so instead of
+# starting a new Claude there (devdesk#38).
+[ -z "$DEVDESK_PUBLIC_URL" ] || url="${DEVDESK_PUBLIC_URL%/}/?arg=$session&existing=1"
 body=$(jq -cMn --arg s "$session" --arg m "$message" --arg u "$url" --arg h "$(hostname)" \
     '{session: $s, message: $m, url: (if $u == "" then null else $u end), host: $h,
       text: ("Claude needs you in " + $s + ": " + $m + (if $u == "" then "" else " — " + $u end))}')