fix(schedule): support systemd 256 missing-unit diagnostic

[xxvcc]

Summary

• accept both exact C-locale missing-unit diagnostics emitted by systemd 255-and-earlier and systemd 256-and-later
• preserve the explicit stop and inactive confirmation before treating a missing timer as cleaned
• cover current and legacy timer namespaces, rollback, active timer, D-Bus, permission, wrong-command, and extra-diagnostic boundaries
• prepare the v2.9.1 changelog and release commands

Testing

• go build ./...; go vet passes with ordinary and integration tags
• gofmt -l . is empty
• Ordinary and race tests pass
• Integration and integration-race tests pass serially in an isolated Debian 13 container
• Staticcheck ordinary and integration tags and govulncheck ./... pass
• bash -n scripts/*.sh, sh -n scripts/install.sh, and shellcheck -S warning scripts/*.sh
• python3 -B -m unittest -v scripts/mirror_receiver_test.py
• Invalid version and tag rejection tests and static amd64 and arm64 builds pass
• actionlint
• Documentation version examples are synchronized; relative links are unchanged
• Debian 13 systemd 257 A/B: public v2.9.0 reproduces the failure; patched binary creates the invite, permits SSH key login and passwordless sudo, activates the timer, and revokes cleanly

Safety Notes

• No private keys, invite bundles, hostnames, server IPs, shadow data, or production authorized keys are committed
• Account, sudoers, and systemd behavior was tested in a disposable Debian 13 environment
• Permission, D-Bus, target mismatch, added output, and uncertain stop state remain fail-closed