detect: skip missing zellij sessions

[dapi]

What changed

• Add zellij.IsSessionNotFound to identify zellij diagnostics where list-sessions reports a session but list-panes says it is not found.
• Make sessions detect skip that disappeared/exited session and continue with the remaining live sessions.
• Keep missing sessions out of the live-session snapshot so existing active records can become stale correctly.

Why

Current zellij can include EXITED - attach to resurrect sessions in list-sessions --short --no-formatting. Calling list-panes for those names returns a session-not-found diagnostic, which made the entire sessions detect fail before it could inspect the real live Codex panes.

Validation

• make
• go test ./...
• git diff --check
• rg -n "zelima|Zelima" . returned only intentional guard/glossary mentions
• Real check: ./zelma sessions detect && ./zelma sessions list no longer fails when zellij reports an exited mercury session