feat(cli): add 'session cleanup' to purge dead, stale sessions

[DoozyX]

Stacked on #1600 — session cleanup needs tmux.ListSessionNamesOnSocket from that PR, so this branch includes its commit. The diff to review here is the cmd/agent-deck/ files; I'll rebase once #1600 lands.

What

session cleanup purges sessions whose tmux pane is gone and that have been untouched past a threshold. It also routes the existing bulk deletes (session remove --all-errored and friends) through the same single implementation, so the delete paths can't drift apart — previously each had its own notion of what was safe to remove.

Why the liveness probe is per-socket

This is the part worth reviewing carefully, because it's a delete path.

Liveness is probed per socket via ListSessionNamesOnSocket. A flat, name-keyed probe built from a single socket reports sessions living on an isolated socket as missing — and in a delete path, "missing" means purging live work. That's a data-loss bug, not a cosmetic one.

Two safety properties:

• Liveness is resolved per socket, so isolated-socket sessions are visible.
• If a socket's probe fails, every session on it is treated as alive and never purged. For an irreversible operation the indeterminate case must fail toward keeping data.

Tests

cmd/agent-deck passes with -race, including new coverage in session_cleanup_cmd_test.go and extended session_remove_kill_test.go.

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📥 Commits

Reviewing files that changed from the base of the PR and between 350a640 and c0d2af7.

📒 Files selected for processing (5)

• cmd/agent-deck/session_cleanup_cmd.go
• cmd/agent-deck/session_cleanup_cmd_test.go
• cmd/agent-deck/session_cmd.go
• cmd/agent-deck/session_remove_cmd.go
• cmd/agent-deck/session_remove_kill_test.go

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