feat(todos): per-todo validation timeout overrides the 20s default

[NIU-123370]

Summary

Lands the second P2 flagged in the #3142 review: the completion-validation gate ran every declared validation_command with a fixed 20s module constant, so tests could only exercise the timeout path by monkeypatching the constant.

• todo add accepts an optional --validation-timeout-seconds alongside --validation-command. Range-checked to 1–29 at the writer side: a timed-out validation must still produce a typed receipt before the outer CLI/MCP subprocess budget (30s) kills the call. Declaring a timeout without a command is rejected.
• The value persists in the todo metadata block and flows through _read_declared_validation into run_completion_validation_gate, replacing the constant in both the runner call and the timeout receipt summary ("timed out after Ns" now reports the declared value).
• Unchanged elsewhere: todos without the field keep the 20s default, the no-command fast path is untouched, and the receipt still captures no stdout/stderr/local paths.

Verification

• pytest tests/control_plane/test_todo_completion_validation.py — 10 passed (3 new: declared 1s timeout cuts off a sleeping command and reports after 1s; out-of-range 30 rejected; timeout-without-command rejected). The override test needs no monkeypatch — the declared value itself drives the timeout.
• all todo-related suites (14 files) — 152 passed
• pytest tests/canary/ (incl. maintainability ratchet) — 20 passed; loopx/todos.py baseline bumped 2165 → 2190 (honest bump for the new flag plumbing)
• mypy — the 4 pre-existing loopx/todos.py errors are identical to main; the new plumbing adds none

Refs #3142 (second P2 follow-up; the first, the module extraction, landed in #3209).

[huangruiteng]

评审结论：通过（Approve）

实现干净、边界合理：#3142 review 标记的第二个 P2 完成——todo add 新增 --validation-timeout-seconds（1–29，writer 侧强校验，声明 timeout 必须同时声明 validation_command），值持久化到 todo metadata 并流经 run_completion_validation_gate，替换 20s 模块常量；超时 receipt 的 summary 现在报告实际使用的声明值。无字段时保持 20s 默认，fast path 与隐私边界不变。

验证（exact head 3a0d1a870，独立 worktree，PR 实际 diff 7 文件 +130/−22）：

• pytest tests/control_plane/test_todo_completion_validation.py tests/canary/：30 passed（含 3 个新用例：1s 超时覆盖、30 越界拒绝、无 command 拒绝）
• git diff --check：无输出
• 新参数导入与上限常量验证正常
• 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。