refactor(todos): extract completion-validation orchestration into a bounded module

[NIU-123370]

Summary

Follow-up to #3142, landing the P2 flagged in its review: extract the completion-validation orchestration out of loopx/todos.py into a bounded module.

• New loopx/control_plane/todos/completion_validation.py (187 lines) holds the read → run → gate sequence behind a single public entry point, run_completion_validation_gate.
• loopx/todos.py delegates to the new module — call sites and receipt semantics unchanged; the file goes 2316 → 2165 lines.
• tests/control_plane/test_todo_completion_validation.py only updates imports.
• Canary baseline ratchets loopx/todos.py lines 2167 → 2165 (re-verified against current main 64c0448c after re-basing; the −2 drift vs. the original stacked branch comes from main, not this change).

Pure move — no behavior change.

Verification

• pytest tests/control_plane/test_todo_completion_validation.py — 7 passed
• pytest tests/canary/ (incl. maintainability ratchet) — 20 passed
• all todo-related suites (14 files) — 149 passed
• mypy on the new module — 0 errors; loopx/todos.py carries the same 4 pre-existing errors as main (zero new)

Refs #3142 (follow-up to #3082).

[huangruiteng]

评审结论：通过（Approve）

纯重构，无行为变更：#3142 review 标记的 P2 完成——把 loopx/todos.py 中的 completion-validation 编排（读取声明命令 → 在 goal repo workspace 运行 → 超时/缺 workspace/命令异常均返回隐私安全的 typed receipt → 失败阻断 completion）整体抽取到新模块 loopx/control_plane/todos/completion_validation.py，对外只暴露 run_completion_validation_gate。调用点与失败语义保持不变；loopx/todos.py 2318 → 2165 行，canary baseline 同步更新。

验证（exact head 38649e76d，独立 worktree）：

• pytest tests/control_plane/test_todo_completion_validation.py tests/canary/：27 passed（含 maintainability ratchet）
• git diff --check：无输出
• 新模块与 loopx.todos 导入正常
• GitHub Actions：build / dependency-review 通过；pytest 正在运行（评审时 pending）

无阻塞问题。CI pytest 转绿后即可合并。