docs: 添加提高 Pester 测试覆盖率的规范文档

新增规范文档，定义将 Pester 测试覆盖率提升至 50% 的目标和要求。文档包括需求说明、任务分解、提案及设计决策，旨在通过增加测试而非排除模块来可靠地提高覆盖率。

Author: mudssky

…/improve-test-coverage-50/.openspec.yaml …-improve-test-coverage-50/.openspec.yamlopenspec/changes/improve-test-coverage-50/.openspec.yaml renamed to openspec/changes/archive/2026-02-11-improve-test-coverage-50/.openspec.yaml openspec/changes/improve-test-coverage-50/.openspec.yaml renamed to openspec/changes/archive/2026-02-11-improve-test-coverage-50/.openspec.yaml

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+## ADDED Requirements
+
+### Requirement: Coverage target
+The system SHALL reach at least 50% command coverage for the Pester suite.
+
+#### Scenario: Coverage measurement
+- **WHEN** tests are run with coverage enabled
+- **THEN** the reported coverage is 50% or higher
+
+### Requirement: Coverage increases come from tests
+The system SHALL improve coverage by adding or expanding tests rather than excluding modules from coverage.
+
+#### Scenario: Coverage improvement approach
+- **WHEN** coverage improves toward the 50% target
+- **THEN** the improvement is attributable to additional tests for existing modules