perf: fix memory leaks — strip task outputs from SSE broadcasts, use light state everywhere

- TaskTracker.getTaskTreeLight(): strips large `output` strings from tasks
  in SSE broadcasts (was serializing 5-10MB every 500ms with many subagents)
- session:created broadcasts now use toLightDetailedState() (consistent with
  session:updated which already did)
- GET /api/sessions/:id returns light state (no 2-3MB terminal+text buffers)
- Ralph wizard polls /terminal?tail=2048 instead of full session endpoint

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/session.ts

@@ -789,9 +789,9 @@ export class Session extends EventEmitter {
         maxTextOutput: MAX_TEXT_OUTPUT_SIZE,
         maxMessages: MAX_MESSAGES,
       },
-      // Background task tracking
+      // Background task tracking (light tree strips large output strings)
       taskStats: this._taskTracker.getStats(),
-      taskTree: this._taskTracker.getTaskTree(),
+      taskTree: this._taskTracker.getTaskTreeLight(),
       // Token tracking
       tokens: {
         input: this._totalInputTokens,