fix(mcp): improve stdio command lookup and error guidance

[LiusCraft]

Summary

• Improve MCP stdio process startup by resolving executable paths from merged PATH entries (explicit Env PATH, system PATH, and common macOS/bin defaults).
• Ensure spawned child processes inherit a deterministic merged PATH while still preserving user-defined non-PATH env vars.
• Add clearer NotFound guidance for common misconfiguration cases (arguments entered in Command vs missing executable/PATH).
• Add unit tests for PATH priority, command resolution behavior, and user-facing spawn error messages.

Testing

• Not run (not requested)

[opencode-agent]

The full build fails due to missing system dependencies (ALSA), but I can analyze the code directly. Here's my review:

Review Summary

Correctness Issues

1. PATH resolution doesn't verify execute permissions (mod.rs:975-982)

   • resolve_stdio_command_path only checks is_file(), not whether the file is executable
   • A non-executable file could be returned, causing spawn to fail anyway

2. Hardcoded Unix paths missing Windows support (mod.rs:928-935)

   • Only macOS/Unix paths are hardcoded (/opt/homebrew/bin, /usr/bin, etc.)
   • No fallback for Windows (C:\Windows\System32, etc.)

3. Test cleanup doesn't propagate errors (mod.rs:1289-1290)

   • Uses let _ = which silently ignores cleanup failures; minor but could mask issues

Missing Tests

1. command_looks_like_path - No direct test for path detection logic
2. Absolute path handling - No test that absolute paths pass through unchanged
3. User env preservation - No test verifying non-PATH env vars are passed to child process
4. Empty PATH edge case - No test for empty/null PATH scenarios

Code Quality

• Good: PATH priority is correct (explicit env → system → fallbacks)
• Good: Deduplication preserves first occurrence (correct priority)
• Good: Error messages are helpful and actionable

Recommendations

1. Add +x permission check or use std::process::Command::new which handles this
2. Consider conditional Windows paths via cfg!(target_os = "windows")
3. Add tests for edge cases mentioned above

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