fix: replace fragile pane grep with Stop hook for idle detection (#272)

## Summary

- Claude Code v2.1.79 permanently shows `❯` in the input area even while actively thinking, causing `monitor_phase_loop` to false-positive on idle detection and kill working sessions after 90 seconds
- Replace `tmux capture-pane | grep ❯` with a Claude Code Stop hook (`lib/hooks/on-idle-stop.sh`) that writes a marker file only when Claude actually finishes responding
- Hook is installed per-worktree in `.claude/settings.json` by `create_agent_session`; marker cleaned up on inject/kill

## Test plan

- [x] Verified hook installs correctly in fresh worktree
- [x] Verified marker file appears only after Claude finishes responding (not during active thinking)
- [x] Verified live dev-agent session picks up fix and Claude works without being killed
- [x] Verified `agent_inject_into_session` clears marker before new work

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: openhands <openhands@all-hands.dev>
Reviewed-on: https://codeberg.org/johba/disinto/pulls/272

lib/hooks/on-idle-stop.sh

@@ -0,0 +1,14 @@
+#!/bin/bash
+# on-idle-stop.sh — Stop hook for dark-factory agent sessions.
+#
+# Called by Claude Code when it finishes a response. Writes a timestamp
+# to a marker file so monitor_phase_loop can detect idle sessions
+# without fragile tmux pane scraping.
+#
+# Usage (in .claude/settings.json):
+#   {"type": "command", "command": "this-script /tmp/claude-idle-SESSION.ts"}
+#
+# Args: $1 = marker file path
+
+cat > /dev/null  # consume hook JSON from stdin
+[ -n "${1:-}" ] && date +%s > "$1"