Prepare v2.10.8: lifecycle fixes and internal simplification

[xxvcc]

Prepare v2.10.8 with fixes for cleanup retries, persistent timers, visible install/uninstall outcomes, mirror scan budgets and cancellation, Linux executable metadata, and init detection. An absent account retains its scheduled retry until database and grant cleanup succeeds; the menu exits once uninstall visibly removes its executable, including durability failures.

Remove unused lifecycle APIs and state, group invite inputs, split large files within their existing packages, and move release-tool tests into a dedicated test package. Preserve CLI options, state formats, backend compatibility, signing-key rotation, and recovery checks. Update both operator-guide languages and the v2.10.8 changelog.

Validation at c2bb291:

• Ordinary and race tests passed as an unprivileged user and in a disposable root integration container. Each root run passed all 993 top-level tests (2,335 including subtests), with no test skips.
• Build, vet, staticcheck, gofmt, module verification, vulnerability scanning, 49 receiver tests, shell syntax, ShellCheck, actionlint and generated primitive checks passed. No vulnerable imported packages or reachable symbols; the module-only OpenPGP advisory concerns an unused package.
• Two independent source archives and build caches produced byte-identical static linux/amd64 and linux/arm64 v2.10.8 binaries. Native amd64 and QEMU arm64 version probes passed.
• The actual unsigned v2.10.8 amd64 candidate passed Debian 13 systemd/SSH/PTY acceptance: five cancellation cases, invitation and persistent timer binding, no-sudo login, immediate SSH denial after revoke, automatic quarantine finalization, and uninstall restoring all six account databases.
• All seven required GitHub checks and the additional CodeQL check passed.

This PR remains a draft for release preparation. The local binaries are unsigned candidates, not official release assets. Signed-tag CI comparison, offline signing, immutable publication, public mirror validation and public upgrade acceptance remain part of the later release process.