refactor(agents): remove terminated AgentState; collapse to paused/error

Drops "terminated" from AGENT_STATES. The agent lifecycle now runs through
idle | active | running | paused | error. paused (carrying a pauseReason)
absorbs every former terminated use case — manual stop, heartbeat run
termination, spawned-child cleanup. Run status (agentRuns.status) is
unchanged: "terminated" stays a valid run-status value.

AGENT_VALID_TRANSITIONS allows direct any→idle transitions so resetAgent
no longer needs the intermediate hop.

Stack-wide:
- core/agent-store: lastError clearing + resetAgent simplified.
- engine/agent-heartbeat, executor, in-process-runtime: terminated state
  writes → paused; halt-state listener fires on paused/error.
- dashboard: AgentsView/AgentListModal/AgentDetailView lose the Terminated
  badge/option/state-block; agent pickers no longer filter terminated;
  agentHealth drops the Terminated branch; routes/state cast widened to
  the new AgentState union.

Tests across core and engine updated to assert paused for AgentState and
left "terminated" intact for run-status assertions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/engine/src/__tests__/heartbeat-executor.test.ts

@@ -291,15 +291,15 @@ describe("executeHeartbeat", () => {
       expect(mockedCreateFnAgent).not.toHaveBeenCalled();
     });
 
-    it("completes with invalid_state when agent state is terminated", async () => {
-      const store = createStoreWithAgentForExec({ state: "terminated" });
+    it("completes with invalid_state when agent state is paused", async () => {
+      const store = createStoreWithAgentForExec({ state: "paused" });
       const monitor = new HeartbeatMonitor({ store, taskStore: mockTaskStore, rootDir: "/tmp" });
 
       const result = await monitor.executeHeartbeat({ agentId: "agent-001", source: "on_demand" });
 
       expect(result).toBeDefined();
       expect(result.status).toBe("completed");
-      expect(result.resultJson).toEqual({ reason: "invalid_state", state: "terminated" });
+      expect(result.resultJson).toEqual({ reason: "invalid_state", state: "paused" });
       expect(mockedCreateFnAgent).not.toHaveBeenCalled();
       expect(store.updateAgentState).not.toHaveBeenCalledWith("agent-001", "active");
     });