refactor: simplify fspy::Command API and add pre_exec hook

[branchseer]

TL;DR

Refactored the fspy crate to simplify the API by removing the Spy struct and making Command the primary entry point.

What changed?

• Removed the Spy struct and replaced it with a global SPY_IMPL static using LazyLock
• Made Command::new() the primary entry point for creating commands instead of Spy::new_command()
• Renamed SpyInner to SpyImpl and moved spawn implementation into this struct
• Added pre_exec support to Command on Unix platforms
• Made most Command fields private with proper accessors
• Added derive_more dependency for better Debug implementations
• Updated all tests and examples to use the new API

How to test?

• Run the existing test suite to verify the refactored API works correctly
• Try creating and running commands with the new API:

  let mut command = fspy::Command::new("program");
  command.args(["arg1", "arg2"]);
  let child = command.spawn().await?;

Why make this change?

This refactoring simplifies the API by removing an unnecessary abstraction layer. The Spy struct was mostly a wrapper around creating commands, and having a global instance was redundant. By making Command the primary entry point and using a global implementation internally, the API becomes more intuitive and follows patterns similar to standard libraries. The change also improves encapsulation by making fields private and adding proper accessors.

[branchseer]

• refactor: simplify fspy::Command API and add pre_exec hook #23 👈 (View in Graphite)
• main

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