Allow concurrency 0 to pause a terminal

A paused terminal gets no new runs and is not probed; in-flight runs still finish.

tests/test_core.py

@@ -160,6 +160,14 @@ class CoreTest(unittest.TestCase):
         self.assertEqual(started, ["GW-1-1", "GW-3-1"])  # 2 slots; GW-2 waits for the game group
         self.assertEqual([self.p.state_of(x) for x in (a, b, c, d)], ["In Progress", "Todo", "In Progress", "Todo"])
 
+    def test_concurrency_zero_pauses_terminal(self):
+        self.h.cfg.terminals["win10"]["concurrency"] = 0
+        w = self.p.add_issue(1)
+        o = self.p.add_issue(2, assignee="oregon")
+        self.h.tick()
+        self.assertNotIn(("win10", "codex"), self.h.execs)  # not even probed (busy_processes)
+        self.assertEqual((self.p.state_of(w), self.p.state_of(o)), ("Todo", "In Progress"))
+
     def test_label_routing(self):
         self.p.add_issue(1, labels=["model:astra"])
         self.p.add_issue(2, assignee="oregon", labels=["cap:gpu"])  # oregon lacks the capability