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/dashboard/app/components/InlineCreateCard.tsx

@@ -926,7 +926,7 @@ export function InlineCreateCard({
                 <div className="dep-dropdown agent-picker-dropdown" onMouseDown={(e) => e.preventDefault()}>
                   <div className="dep-dropdown-search-header">Select agent</div>
                   {agentsLoading && <div className="dep-dropdown-empty">Loading agents...</div>}
-                  {!agentsLoading && agents.filter((a) => a.state !== "terminated").map((a) => (
+                  {!agentsLoading && agents.filter((a) => true).map((a) => (
                     <div
                       key={a.id}
                       className={`dep-dropdown-item${selectedAgentId === a.id ? " selected" : ""}`}
@@ -941,7 +941,7 @@ export function InlineCreateCard({
                       <span className="dep-dropdown-title">{a.name}</span>
                     </div>
                   ))}
-                  {!agentsLoading && agents.filter((a) => a.state !== "terminated").length === 0 && (
+                  {!agentsLoading && agents.filter((a) => true).length === 0 && (
                     <div className="dep-dropdown-empty">No agents available</div>
                   )}
                   {selectedAgentId && (