feat(ckpt): make diff -t optional to compare snapshot against current workspace

[Ziqi002]

Description

• diff 命令的 -t/--to 参数改为可选，省略时与当前工作区（live subvolume）进行比较 ([ckpt] feat: diff -t 改为可选，支持快照与当前工作区比较 #846)

改动范围

从 CLI 到 daemon 到插件全栈适配：

协议层：Request::Diff.to 从 String 改为 Option<String>，
StorageBackend::diff() 的 to 参数从 &str 改为 Option<&str>。

CLI：clap 的 -t 参数从 required 改为 optional。

daemon：snapshot_mgr 和 dispatcher 适配 Option 传递。
两个 backend（btrfs_base、btrfs_loop）在 to 为 None 时
使用 data_root.join(ws_id) 作为 live subvolume 路径。

插件：hermes（Python）和 openclaw（TypeScript）的 tool schema
中 to 从 required 移除，命令构建改为有条件添加 --to。

Related Issue

closes #846

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