fix(ckpt): sync SKILL.md with actual CLI/plugin implementation

[Ziqi002]

Description

• 修正 rollback -w 参数描述：标为必填，与实际 CLI 定义一致
• 新增 diff 命令文档（触发表、用法、参数说明）
• diff -t 标注为可选，省略时与当前工作区状态比较（对齐 [ckpt] feat: diff -t 改为可选，支持快照与当前工作区比较 #846 的需求）
• 修正 typo：workspacee、未闭合 <path-to-workspace 标签

SKILL.md 存在多处与实际 CLI/插件行为不一致的描述，本 PR 统一修正并补全缺失的 diff 命令文档。

Related Issue

closes #799

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional change)
• Performance improvement
• CI/CD or build changes

Scope

• cosh (copilot-shell)
• sec-core (agent-sec-core)
• skill (os-skills)
• sight (agentsight)
• tokenless (tokenless)
• memory (agent-memory)
• ckpt (ws-ckpt)
• Multiple / Project-wide

Checklist

• I have read the Contributing Guide
• My code follows the project's code style
• I have added tests that prove my fix is effective or that my feature works
• I have updated the documentation accordingly
• For cosh: Lint passes, type check passes, and tests pass
• For sec-core (Rust): cargo clippy -- -D warnings and cargo fmt --check pass
• For sec-core (Python): Ruff format and pytest pass
• For skill: Skill directory structure is valid and shell scripts pass syntax check
• For sight: cargo clippy -- -D warnings and cargo fmt --check pass
• For tokenless: cargo clippy -- -D warnings and cargo fmt --check pass
• For memory (Linux only): cargo clippy --all-targets -- -D warnings, cargo fmt --check, and cargo test pass
• Lock files are up to date (package-lock.json / Cargo.lock)

Testing

Additional Notes