fix(status): bound agent todo index output

[huangruiteng]

Summary

• keep the whole-goal todo_index.items detail off the status --agent-id hot path while preserving the index schema, counts, and a cold-path read hint
• retain full todo-index detail for operator status without --agent-id and for todo list
• tighten the characterized status JSON size, line, and per-todo growth ceilings

Evidence

• real loopx-meta agent-scoped status: 269,923 -> 176,592 bytes (34.6% reduction); 5,570 -> 3,535 lines
• CLI output budget suite: 8 passed
• Ruff 0.12-0.15 policy: passed
• risk-based premerge: 17/17 selected checks passed, 0 failures, 0 manual holds

Boundary

Public control-plane code and deterministic fixture coverage only. No private state, raw run evidence, credentials, local paths, or generated logs are included.