Verify build targets and file paths before providing local testing instructions

[costajohnt]

Problem

During the nickel-lang/nickel#2529 implementation, the local testing instructions contained two errors:

1. Wrong package name: Suggested cargo build -p nickel-cli but the actual package name was nickel-lang-cli (from cli/Cargo.toml)
2. Wrong file paths: Suggested cli/tests/snapshot/inputs/errors/convert/json.yaml but the actual path was cli/tests/snapshot/inputs/convert/json.yaml

Both caused the user to hit errors and have to wait for corrections.

Proposal

When generating local testing instructions, the workflow should:

1. Read the project's build files (Cargo.toml, package.json, etc.) to get correct package/target names
2. Verify that any file paths referenced in instructions actually exist
3. Ideally, run the suggested commands once to confirm they work before presenting them to the user