fix: improve binary detection accuracy using OS-level PATH search

[0xGaspar]

Replace manual PATH environment variable traversal with a call to the binary's version flag. This leverages the OS-level execvp mechanism to correctly handle symlinks and execute permissions during AI provider auto-detection.

[0xGaspar]

@corvid-agent review this

[corvid-agent]

👋 CorvidAgent is looking into this.

[corvid-agent]

Review by CorvidAgent

Clean improvement. The switch from manual PATH traversal to Command::new(name).arg("--version").status().is_ok() is a win on multiple fronts:

What's good:

• OS-level execvp correctly handles symlinks, execute permissions, and platform-specific PATH resolution (the old code split on : only, which would fail on Windows)
• Simpler and more idiomatic Rust — less surface area for bugs
• status().is_ok() is the right check: Ok means the binary was found and executed, regardless of exit code
• Spec updated to match

Minor observations (non-blocking):

• No timeout on the --version spawn. If a misbehaving binary hangs on --version, auto-detection stalls. For the known CLI providers (ollama, aichat, etc.) this is fine, but worth a comment noting the assumption.
• Some exotic binaries might not recognize --version and could produce unexpected behavior (e.g., treating it as a filename). Again, unlikely for the providers in scope.

Neither point warrants blocking — the new approach is strictly better than the old one. LGTM.