…ys which one wants you

Implements PRD 0009. `moshcode start claude -d` now runs the engine in a
runtime that outlives the pit and hands the prompt straight back, so the
pit stops being a one-thing-at-a-time shell: `/ps` shows what is running
and what state each session is in, `/attach` steps into one, detaching
leaves it running.

Two substrates behind one interface, because "requires tmux" was the wart
worth removing rather than documenting:

  tmux  a single named server (socket `moshcode`, not tabs.mjs's per-pid
        one). Full fidelity — resizing, scrollback, native attach.
  pty   no tmux: script(1) allocates the pty, the child is detached with
        its stdin on a FIFO held O_RDWR so it never sees EOF when the pit
        exits, and attach replays the transcript and relays keystrokes.
        Sized from inside via stty; a later resize cannot reach it.
  none  foreground passthrough, exactly as today, said once.

Semantic state (working/blocked/done/idle/unknown) with herdr's rule that
each session has ONE authority: a live hook report suppresses screen
classification entirely, and expires so a crashed agent cannot read
`working` forever. Screen rules ship beside each engine's install spec,
are anchored to things a terminal draws rather than English words, and
are overridable in ~/.moshcode/herd/rules.json when they rot.

`blocked` can page the operator through the existing notify()/ask()
fan-out, with the reply typed back into the session that was waiting —
the part herdr structurally cannot do. Only transitions notify, so a
session sitting blocked does not page every five seconds.

One surface for humans and agents: every verb takes --json, `wait` exits
0/2/3 so scripts can branch, and moshscript gets herdStart/herdPrompt/
herdWait/herdRead/herdList/herdKill as values rather than exit codes.

Two bugs the survival test caught, both of which would have shipped as
"finished agents report gone": tmux's remain-on-exit was set in a second
call that a fast command could beat, now one invocation via tmux's `;`;
and the pty substrate could not tell a finished agent from a rebooted
box, now the session's own shell records its exit.

Named `herd`, not the `runtime`/`agent <verb>` the PRD first proposed —
`agent` is already an alias of `agents` and a test pins that, and
`runtime` is what src/runtime.mjs already means. PRD updated to match.

Deferred and stated in the PRD: auto-installing the status hook into each
engine (the protocol ships and works), pointing ttyd at a real session,
and scrollback replay across reboots.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

…node exit under it

CI caught this; a green local suite did not. Thirteen tests in
herd-cli.test.mjs came back `cancelledByParent`, which was the test
runner's way of reporting that the file's process had died mid-run.

The cause is a real bug in shipped behaviour, not a test artifact. The
poll timer in waitFor() was unref'd, copying the instinct from mirror.mjs
where a background nicety must never hold the process open. Here that is
exactly backwards: `wait` and `watch` exist to keep the process alive. An
unref'd timer means node finds nothing scheduled between polls and exits.
It does not hang — it is worse. `moshcode wait api --timeout 1h` returned
in a millisecond, exit 0, having waited for nothing, and any script
branching on it would have read that as "the agent is ready".

Measured before: waitFor resolved never and the process exited after 1ms.
After: it waits the full duration and settles. End to end, `moshcode wait`
now blocks the whole timeout and exits 2, exits 0 the moment the state
matches, and exits 3 for a session that does not exist.

Also un-unref's the liveness poller in the pty substrate's attach, which
was the same trap one step from being sprung: the follow timer beside it
is already unref'd, so an attach whose stdin did not hold the loop open
would have exited the instant it started. It is cleared on detach, so it
never outlives the attach either.

The regression test runs waitFor in its own process with nothing else
pending, because the test runner itself keeps the event loop alive and
hides the bug completely. Verified it fails against the old code before
keeping it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>