fix(shell): explicitly default to environment cwd when None

[Copilot]

The Shell tool's cwd parameter is optional and should default to the environment's current working directory when not specified. The previous implementation used PathBuf::from(".") as an intermediate value, relying on subsequent path normalization to resolve to the environment cwd.

Changes

• Direct environment cwd access: Changed from PathBuf::from(".") to self.services.get_environment().cwd.clone() when input.cwd is None
• Added clarifying comment: Documents the default behavior inline

Before/After

// Before: indirect via path normalization
let cwd = input.cwd.unwrap_or_else(|| PathBuf::from("."));
let normalized_cwd = self.normalize_path(cwd.display().to_string());

// After: explicit environment cwd default
let cwd = input
    .cwd
    .unwrap_or_else(|| self.services.get_environment().cwd.clone());
let normalized_cwd = self.normalize_path(cwd.display().to_string());

Behavior unchanged—improves code clarity and intent.

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