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

hive-subagent-mcp/Cargo.toml

@@ -24,6 +24,12 @@ tokio.workspace = true
 tracing.workspace = true
 tracing-subscriber.workspace = true
 
+# `test-util` for `#[tokio::test(start_paused = true)]`: the `continue`
+# resume-grace tests assert what happens when the bound is actually reached,
+# and paused time gets that answer without a five-second unit test.
+[dev-dependencies]
+tokio = { workspace = true, features = ["test-util"] }
+
 # `hive-subagent-daemon` — long-running per-agent claude-subagent runner.
 # Independent of `hive-bash-mcp` (own crate, own binary, own MCP server) —
 # see lib.rs's module doc for why. Serves its MCP tools (`start`/