subagent: make a missed continue the tool call's own error

`continue` returned "started" the instant `Claude::spawn` handed back a
pid, and a resume that matched nothing only surfaced later, as an
end-of-turn todo. By then the caller had moved on believing it had a
running subagent.

A pid is proof enough for `start`, which creates its session: the spawn
succeeding is the whole story. It is not proof for a resume — claude
exits non-zero a fraction of a second *after* the process exists. So
`continue` now waits for the first real answer and reports a miss as its
own `Err`, carrying claude's message and the directory searched.

The wait ends on whichever comes first, so a successful `continue` pays
no fixed delay: the turn's first non-terminal stream event settles it at
about the same moment a miss's exit would have. Measured on this box:
14 runs of the driver's own invocation against a missing session took
550-1087 ms spawn to exit, and a healthy turn's first event lands at
roughly 500 ms. The five-second cap is ~4.6x the slowest miss and is only
ever reached by a child that neither speaks nor exits.

The underway signal reads the event's kind, not its content: a missed
resume is not silent — it emits a terminal `result` event and stderr
before exiting — so "any sink callback" would have reported every miss as
a successful start. Liveness still counts all three callbacks.

The end-of-turn todo is unchanged for every failure later in the turn;
the only one it no longer repeats is the miss the caller was just handed.

Refs #4405

claude-plugins/plugins/base/skills/claude-subagents/SKILL.md

@@ -30,9 +30,13 @@ status(name)
 interrupt(name, force?)
 ```
 
-`start` and `continue` return as soon as the process is confirmed
-running, not once it finishes — a completion lands as a todo
-(`get_loose_ends`), same as any other producer. Use `status` for a
+`start` and `continue` return once the turn is under way, not once it
+finishes — a completion lands as a todo (`get_loose_ends`), same as any
+other producer. `continue` takes the extra moment to confirm the resume
+actually attached, so a `continue` naming a session that isn't there
+fails the tool call outright instead of looking like it worked; the error
+says which directory it searched, which is usually the fix (pass `dir`).
+Use `status` for a
 zero-cost "is it still going" check — for a running subagent it also
 reports how long since that turn last produced output, so a few seconds
 means it's working and an age climbing into the minutes means it's