feat(agent): overlay GC and mount namespace cleanup

[miguelgila]

Summary

• Overlay GC: Reconciles on-disk overlay namespaces against the Kubernetes API, removing overlays for namespaces that no longer exist (merged from previous commits)
• Mount namespace cleanup: Detects and removes stale /run/reaper/ns/* bind-mount files that occur when helper processes crash, nodes partially reboot, or overlay dirs are manually cleaned
• Deploy fix: Adds mountPropagation: HostToContainer to the agent's run-reaper volume mount for mount-point visibility

Mount namespace cleanup details

• Detection via /proc/self/mountinfo with /proc/1/mountinfo fallback (hostPID)
• Non-blocking flock prevents races with the runtime's enter_overlay()
• Running container safety check prevents removing ns files for active workloads
• Runs before each overlay GC pass, piggybacks on existing --overlay-gc-enabled and --overlay-gc-interval flags
• New metrics: reaper_agent_ns_cleanup_runs_total, reaper_agent_ns_cleaned_total

Test plan

• cargo clippy --target x86_64-unknown-linux-gnu --all-targets — clean
• cargo test — all unit tests pass
• ./scripts/run-integration-tests.sh --agent-only — 13/13 agent tests pass
  • Agent ns cleanup stale file — verifies stale ns files are removed
  • Agent ns cleanup preserves active — verifies running container protection
  • Agent ns cleanup metrics — verifies metrics are exposed

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.83%. Comparing base (78dae3a) to head (63d981f).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
- Coverage   87.18%   86.83%   -0.36%     
==========================================
  Files           6        6              
  Lines         320      319       -1     
==========================================
- Hits          279      277       -2     
- Misses         41       42       +1     

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