fix: improve session expiry and headless diagnostics

[kongesque]

Summary

• improve expired-session handling and related CLI behavior
• improve headless/systemd-credentials diagnostics and storage-control handling
• add regression coverage for session manager and Linux headless credential flows
• update CLI, protocol, headless, and localized README documentation

Scope

This PR merges feat/session-expiry-headless-diagnostics into main.

[coderabbitai]

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