restore, step 2: resume agents, retype command lines

- the pi extension reports its session file; the daemon keeps it in the
  pane option @agent_session, and snapshots carry it. restore runs
  `pi --session <file>` in the rebuilt pane, one agent every 0.5s, then
  reports which agents run and which were interrupted mid-turn.
- other panes: the command line running in them is typed at the new
  prompt but not run. exactly as typed with the zsh hook (dingger install
  zsh: preexec/precmd keep @dingger_cmd), else the foreground program's
  exact argv from the kernel (KERN_PROCARGS2), shell-quoted.
- "at a prompt" is the pane's own shell owning the terminal's foreground
  process group, not a name check: `sh -c ...` is a running program.
- an agent or command that exits keeps its place in snapshots for 60s, so
  a shutdown that stops programs before tmux still restores them.
- typing waits for each new prompt (text typed earlier echoes twice).

e2e step 16 covers resume with a session path containing a space, the
interrupted report, an agent that exits just before the server dies, a
quoted command retyped not run, and the hook's text; dropping the grace
period, judging prompts by name, running retyped commands, or not storing
the session reference each fail it. the test's fake pi is guarded so a
real pi never starts.

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