Prune stale log files on startup

[timhartmann7]

The daily rolling appender creates one omnyssh.log.YYYY-MM-DD file per day in the config directory and never cleans them up, so logs accumulate indefinitely and fill the disk over time.

This adds cleanup_old_logs, which runs once at startup and removes rolling log files older than 7 days. It is:

• Cross-platform. Operates on dirs::config_dir()/omnyssh, the same directory used on macOS, Linux, Termux, and Windows. Age is read from filesystem mtime, so the logic is identical everywhere.
• Fault-tolerant. A missing directory, unreadable entry, or failed delete is skipped rather than propagated, so cleanup can never block startup.
• Safe. Only files prefixed with omnyssh.log are considered; config.toml, hosts.toml, and snippets.toml are never touched.

Covered by 4 integration tests: old files removed, recent files kept, non-log files untouched, and missing directory is a no-op.