Subagents show up under their parent pane (#272)

A subagent running in the background used to vanish from the sidebar once its parent's turn ended, and the pane read "Idle" while the subagent was still working. Now:

• Claude Code: every subagent gets its own line under the pane (├ Explore ⚒3 · Bash) — automatically, no setup. wmux now registers the SubagentStart hook and reads the agent_id Claude Code puts on every hook fired from inside a subagent, so each line ends on its own SubagentStop, not when the parent's turn ends. While one runs, the row reads "Orchestrating · N agents" and the dot stays green.
• Any other agent or harness: declare your own subagents/background tasks with one command:

  wmux report-subagent planner-1 --name planner --detail "calling create_file"   # start / update (also a heartbeat)
  wmux report-subagent planner-1 --done                                          # finish

  Display only — it never marks the pane "Needs you" or clears that.
• A running line that has been silent for 10 minutes stops being drawn, so a lost "done" can't leave a ghost.

Claude Code users: restart wmux once so the new hook gets written to ~/.claude/settings.json.

Full Changelog: v2.18.2...v2.19.0