feat(action): run --validate before generating so bad definitions fail fast

[BryanFRD]

Problem

The generator already ships a validation mode — generator/src/cli.rs exposes --validate (Mode::Validate) and generator/src/validate.rs implements the checks (duplicate package names/paths, hook path traversal, etc., per the closed issues #70/#71). But action.yml never uses it: the "Generate fixtures" step (lines 56-63) jumps straight to generate-fixtures --definitions ... --output ....

Why it matters

When a consumer passes a malformed definitions directory, generation fails partway through with a generation-time error rather than an upfront validation error that names the offending file and the specific problem. Validation exists precisely to give that clear signal, and the CLI flag is documented (README directory-structure note for validate.rs), yet the Action — the primary entry point for consumers — bypasses it.

Proposed approach

• Add a step in action.yml before "Generate fixtures" that runs generate-fixtures --validate --definitions "${{ inputs.definitions }}" and fails the job on a non-zero exit.
• Keep it cheap: same already-built release binary, no extra build.

Acceptance criteria

• action.yml validates definitions before generating.
• A definitions dir that fails validation surfaces the validation error and stops the job before generation runs.