fix: validate --app-config path early with clear error

[nanookclaw]

Closes #42.

When an invalid --app-config (or NEMO_APP_CONFIG) path is passed, the failure currently surfaces deep in the loading pipeline as a generic IO error. This adds a targeted check in crates/nemo/src/main.rs right after Args::parse(), before runtime::NemoRuntime::new(...) and the GPUI early-runtime path, so the user sees what they actually passed.

Behavior:

• Relative paths are resolved against std::env::current_dir() for the exists/is_file checks. If current_dir() fails the path is checked as supplied — the not-found error then reports the original path the user passed, never a canonicalized variant.
• Non-existent paths are never canonicalized.
• Error messages match the issue spec exactly:
  • Error: config file not found: <path>
  • Error: config path is not a file: <path>
  • followed by std::process::exit(1).

The existing if let Some(ref app_config) = args.app_config { ... headless/validate ... } block is left intact; the new block is additive and only changes behavior for paths that would have failed anyway.

Verified with cargo check -p nemo and cargo clippy -p nemo --no-deps (clean).