Sidekick Skill — Persistent Worker Agent for Claude Code

A Claude Code skill for spawning persistent, crash-recoverable worker agents ("sidekicks") that handle long-running missions. The default mode runs the sidekick as a named background Agent inside the invoking session's Agent Team, using STATE.md checkpoints and a resumption bead for durability — if the session crashes, a fresh one respawns from disk state. As a fallback, the sidekick can run in an external tmux session when work must survive the parent CLI exiting entirely; this mode sacrifices team-panel visibility.

The sidekick checkpoints progress to STATE.md every five minutes: a file at /tmp/<project-slug>/sidekick/<mission-slug>/STATE.md holding the mission description, ground-truth invariants, standing rules, an append-only progress log, and the next actions list. Each heartbeat appends a timestamp and updates the resumption bead, capping data loss at five minutes.

Authentication environment variables must be propagated explicitly in the launch command — tmux servers snapshot environment at startup and will not inherit them automatically. Communication from the main session to the sidekick goes through STATE.md plus tmux capture-pane; Agent Teams is one-team-per-session with no cross-session joins, so sidekicks in external tmux sessions are not SendMessage-addressable.

Interactive TUI sidekicks have a documented failure mode of stalling alive (typically from the session-limit modal) rather than exiting. A stall watchdog monitors STATE.md mtime (>15 minutes triggers an alarm) and checks every teammate pane, recovering stalled panes via tmux send-keys. Three drive-loop invariants emerged from incident retro: an endgame single-writer freeze on target files, once-per-head external-bot requests, and deadman timeouts on background waits. The skill defines procedures to migrate sidekicks between in-session and external-tmux modes, with STATE.md as the invariant across transitions. Both Claude and Codex engines are supported, with detailed launch commands for each.