Add cleanup/GC for stale session artifacts (todos, team inboxes, sub-agent state)

Problem

Claude Code accumulates stale artifacts on disk with no built-in cleanup mechanism:

• ~/.claude/todos/ — task files from sub-agents pile up indefinitely. After moderate usage, 80+ orphaned JSON files accumulate.
• ~/.claude/teams/*/inboxes/ — team agent inboxes persist after the team session ends.
• ~/.claude/sessions/ — session files remain even after the owning process dies (e.g., a 14-day-old zombie session with a dead PID).
• Sub-agent autocomplete ghosts — agents spawned in other active sessions (e.g., @scrutinize2-632, @economist-r1, @agent-638) appear in the @ autocomplete across all sessions, with no way to dismiss or clean them up.

There is no claude gc, claude sessions --prune, or equivalent. The only recourse is manually rm-ing files.

Expected behavior

• Session files should be cleaned up when the owning PID is no longer running (on startup or periodically)
• Todo files should be removed when their owning session exits
• Team inboxes should be pruned when no active process references them
• Sub-agent names should be scoped to their parent session and not leak into other sessions' autocomplete
• Ideally: a claude gc or claude clean CLI command for on-demand pruning

Current workaround

rm ~/.claude/todos/*.json
rm ~/.claude/teams/default/inboxes/*.json
# Manually kill stale PIDs from ~/.claude/sessions/

Environment

• macOS (Darwin 25.3.0)
• Claude Code CLI (latest as of 2026-04-07)