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/core/src/agent-store.ts

@@ -1173,7 +1173,9 @@ export class AgentStore extends EventEmitter {
         state: newState,
         updatedAt: new Date().toISOString(),
         // Clear lastError when transitioning away from terminated
-        ...(currentState === "terminated" && newState !== "terminated" && { lastError: undefined }),
+        // Clear lastError when an agent re-enters an actionable state so
+        // a resumed agent does not carry stale "Error" badges.
+        ...((newState === "active" || newState === "running") && { lastError: undefined }),
       };
 
       await this.writeAgent(updated);
@@ -1432,11 +1434,9 @@ export class AgentStore extends EventEmitter {
       await this.endHeartbeatRun(activeRun.id, "terminated");
     }
 
-    // Normalize to terminated first when idle is not directly reachable.
-    if (agent.state !== "idle" && agent.state !== "terminated") {
-      agent = await this.updateAgentState(agentId, "terminated");
-    }
-
+    // Any non-idle state can transition directly to idle in the new
+    // lifecycle (see AGENT_VALID_TRANSITIONS in types.ts), so no
+    // intermediate hop is required.
     if (agent.state !== "idle") {
       agent = await this.updateAgentState(agentId, "idle");
     }