fix(ssh): add EMFILE backoff and exit notification to SSH accept loop

[politerealism]

Summary

Depends on #2370 — this branch is stacked on fix/proxy-exit-detection. Please review #2370 first; once it merges the diff here will collapse to only the SSH-specific changes below.

Applies the same two-layer defense from the proxy accept loop (#2369 / #2370) to the SSH accept loop:

• EMFILE backoff: classify accept errors as Retry (EMFILE/ENFILE/ENOBUFS — exponential backoff) vs Terminal (≥10 consecutive unknown errors → break). Mirrors classify_accept_error in openshell-supervisor-network.
• Exit notification: thread a oneshot::Sender<()> drop-guard through run_process so the sandbox is notified when the SSH task ends for any reason. The sandbox selects on ssh_exited in both process_enabled paths and returns an error, causing Kubernetes to restart the container rather than leaving it running without SSH access.

Related Issue

Closes #2372

Changes

File	Change
crates/openshell-supervisor-process/src/ssh.rs	Replace bare accept().await? with classify-and-retry loop; add SshAcceptAction enum and classify_ssh_accept_error
crates/openshell-supervisor-process/src/run.rs	Add ssh_exit_tx: Option<oneshot::Sender<()>> parameter; hold as drop-guard in SSH spawn block
crates/openshell-sandbox/src/lib.rs	Create (ssh_exit_tx, ssh_exit_rx) channel when ssh_socket_path is Some, pin ssh_exited future, select on it in both process_enabled paths

Testing

• cargo clippy -p openshell-sandbox -p openshell-supervisor-process — clean
• cargo fmt --check — clean
• Sandbox e2e: verify container exits when SSH accept loop terminates unexpectedly

Checklist

• Follows Conventional Commits format
• DCO Signed-off-by present
• No unrelated changes in SSH-specific commit
• Stacked on fix(sandbox): terminate sandbox when proxy accept loop exits unexpectedly #2370; PR description notes dependency

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[politerealism]

Once #2370 merges into main, this branch will need to be rebased onto the updated main and force-pushed. The rebase should be clean — the proxy changes and SSH changes don't overlap.

[sjenning]

/ok to test 5ed5f7a

[github-actions]

Label test:e2e applied for 5ed5f7a. Open the existing run and click Re-run all jobs to execute with the label set. The run will execute the standard E2E suite after building the required gateway and supervisor images once. The matching required CI gate status on this PR will flip green automatically once the run finishes.