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>

tests/attach.bats

@@ -310,7 +310,7 @@ posts() { [ -f "$TMP/posts" ] && wc -l <"$TMP/posts" || echo 0; }
     sleep 2.5
     [ "$(posts)" -eq 1 ]
     run jq -r '[.session, .url, (.text | test("Claude needs you in alpha"))] | @tsv' "$TMP/posts"
-    [ "$output" = "$(printf 'alpha\thttps://dev.example.org/?arg=alpha\ttrue')" ]
+    [ "$output" = "$(printf 'alpha\thttps://dev.example.org/?arg=alpha&existing=1\ttrue')" ]
 
     # answered within the delay: nothing is posted
     echo '{"hook_event_name":"PostToolUse"}' | hook