fix(skill-mcp-manager): remove process listeners on disconnect and guard connection races

H7: Process 'exit'/'SIGINT' listeners registered per-session were
never removed when all sessions disconnected, accumulating handlers.
- Add unregisterProcessCleanup() called in disconnectAll()

H8: Race condition where disconnectSession() during pending connection
left orphan clients in state.clients.
- Add disconnectedSessions Set to track mid-flight disconnects
- Check disconnect marker after connection resolves, close if stale
- Clear marker on reconnection for same session

Tests: 6 pass (3 disconnect + 3 race)

src/features/skill-mcp-manager/manager.ts

@@ -10,6 +10,7 @@ export class SkillMcpManager {
   private readonly state: SkillMcpManagerState = {
     clients: new Map(),
     pendingConnections: new Map(),
+    disconnectedSessions: new Set(),
     authProviders: new Map(),
     cleanupRegistered: false,
     cleanupInterval: null,