fix(cgroup): skip control files when sweeping stale lineage sessions

[SequeI]

Linked Issue

Closes #1793

Summary

teardown_session_tree read_dir a session cgroup dir and rmdir every entry, including the cgroups own control/pseudo-files (memory.events, pids.peak, io.latency, ...), not just the cmd_* child cgroups it meant to remove. rmdir on those files fails with ENOTDIR, logging spurious "could not remove cgroup leaf" warnings on every stale-session sweep.

Filter to cmd_-prefixed entries only, matching CMD_PREFIX used elsewhere in this file.

Test Plan

Checklist

• An issue exists and is linked above
• All commits are signed-off, using DCO
• All new code follows the project's coding standards (CLAUDE.md) and is covered by tests
• Public-facing changes are paired with documentation updates
• If this PR introduces a major feature, capability, or security-relevant change, a corresponding NEP has been opened or accepted and is linked

[github-actions]

PR Review Summary

Size

Metric	Value
Lines added	+9
Lines removed	-1
Total changed	10
Classification	Small (< 50 lines)

Affected crates

• crates/nono-cli — CLI changes. Verify argument parsing, flag documentation, and UX behaviour across supported platforms.

Blast radius — Contained

This PR touches: source code

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_{Updated automatically on each push to this PR.}

[SequeI]

@icydipity let me know if this works please! Worked on my repro

[nogent-nolabs-ai]

nogent code review

No blocking issues found.

Findings: none flagged in scope.

_{Automated code + security review. CI already covers clippy, rustfmt, tests, cargo-audit and commit-lint.}

[icydipity]

@icydipity let me know if this works please! Worked on my repro

Yes, this worked. Thanks for the fix!