subagent daemon: explain the status when returning it, not up front

The `status` tool description enumerated all five states it can report, so
every caller paid for four answers it didn't get and read the explanation
in the wrong place. The description now states only what the tool is for
and that it costs nothing to call; each answer it returns carries its own
meaning and the caller's next move instead — running, starting and idle
were terse, the not-found error terser still, and they had been leaning on
the enumeration to be legible. The killed answer and the end-of-turn todo
are unchanged. `continue`'s description keeps its killed-resume sentence:
that describes what the tool does, not a state it might hand back.

Refs #4326

hive-subagent-mcp/src/mcp.rs

@@ -164,12 +164,8 @@ impl SubagentMcp {
 
     #[tool(
         description = "Report whether a subagent is currently running — a zero-cost check that \
-            never launches a process, unlike `continue`. Distinguishes running, starting (a \
-            `start`/`continue` is in flight but not yet a confirmed spawn — this is normally \
-            over in well under a second), killed (its last turn died on a signal — the kernel's \
-            OOM killer, a stopped unit, an `interrupt` — so its work was cut off mid-turn rather \
-            than finished), idle (a session exists, its last turn finished, nothing is in flight \
-            — `continue` to give it another turn), and no such session at all."
+            never launches a process, unlike `continue`. The answer says what state it found \
+            and what to do about it."
     )]
     fn status(&self, Parameters(args): Parameters<StatusArgs>) -> String {
         match session::status(&self.state, &args.name, args.dir.as_deref()) {