Sessions leak memory — OOM kills from zombie accumulation

Problem

Sessions that finish, crash, or get abandoned never have their finished_at set in the registry. They stay marked as "status": "running" forever. Over time, the orphaned tmux sessions and their child processes (node, claude) accumulate and eat memory until the OOM killer starts picking off active sessions.

Evidence from today:

• 19 sessions killed with exit code 137 (SIGKILL/OOM) across the registry's lifetime
• At time of the latest kill (neowolt-dusty-branch-a8dc99), 11+ sessions had been spawned in the last 6 hours alone
• Dozens of older sessions still marked "running" with finished_at never set
• Container has 7.7GB RAM, swap was nearly full (877M/1G used)

What needs to happen

1. Session reaper — periodic cleanup that:

• Checks if the tmux session still exists for each "running" registry entry
• If the tmux session is gone, marks it as orphaned/finished with a timestamp
• Optionally kills tmux sessions that have been idle too long (no claude process, no recent activity)

2. Lifecycle hooks — set finished_at and status when:

• Claude process exits (exit code capture already works — exit_code: 137 was recorded)
• tmux session is destroyed
• Session is explicitly stopped

3. Memory budget — optional but useful:

• Cap max concurrent sessions (e.g., reject new spawns if N active)
• Or at minimum, warn when memory is above a threshold before spawning

Where to look

• Registry files: .state/registry/*.json
• Session spawn logic: wherever tmux sessions + claude are launched
• The finished_at field exists in the schema but is only set for some exit paths

Impact

Active sessions get randomly killed mid-conversation. Work is lost, user gets no warning. The killed session today was mid-conversation with the user doing iterative design work.

Start by inspecting .state/registry/*.json to understand the recorded status and finished_at fields, then locate the session spawn logic where tmux and Claude are launched. Trace the existing exit-code handling and explicit stop path. Done means orphaned and completed sessions are finalized, stale sessions are cleaned up safely, and new spawns have the requested memory protection.