Close the review's gaps round the agent account (hq ADR 0266)

The managed settings apply to every account's claude, so a key-helper script under the
agent's home would run as the operator, and through sudo as root: the command is now
carried whole in the root-owned managed settings and reads the key only as the agent.
The helper reads only a regular file, bounded, without blocking, and a request it does
not answer fails in time instead of holding every later one; a directory entry it
answers that is not a name is refused. The guard's record is matched on its binary in
the query, a half-named agent account is not taken as the operator's, the kept API key
stays until the agent's home holds one, restic refuses to restore beside a directory
others may write, and the agent-home check cleans a path before judging it.

modules/claude-code/cmd/claude-code/main.go

@@ -391,7 +391,13 @@ func main() {
 	}
 	if !launched {
 		// Outside a launch — a build, a check — it serves nothing and says why.
-		say("not launched by the runtime with this module's words; serving no tools")
+		if (os.Getenv("MESH_AGENT_ACCOUNT") == "") != (os.Getenv("MESH_AGENT_HOME") == "") {
+			say("the runtime named the agent's account without its home, or its home without the account " +
+				"(MESH_AGENT_ACCOUNT, MESH_AGENT_HOME); serving no tools rather than writing the operator's home " +
+				"for an agent that runs as another account")
+		} else {
+			say("not launched by the runtime with this module's words; serving no tools")
+		}
 		if err := stdio.Serve("", nil); err != nil {
 			os.Exit(1)
 		}