Rename default project file from .struct.yaml to .structkit.yaml

[httpdss]

Issue

The product was originally named STRUCT, so the default project file was .struct.yaml. This aligns that file and its references with the StructKit name.

Description

The canonical project structure file is now .structkit.yaml. Existing .struct.yaml files keep working.

Behavior

• structkit generate (with no structure argument) prefers .structkit.yaml, then falls back to .struct.yaml.
• If both files exist, .structkit.yaml is used.
• structkit init writes .structkit.yaml and skips creation when either the canonical or legacy file is already present.
• The reusable struct-generate GitHub Action detects .structkit.yaml or .struct.yaml when struct_file is omitted, so older CLI versions still work.
• IDE schema mappings include both filenames.

In-repo examples, contribs, docs, and prompts now use .structkit.yaml.

Checklist

• I have read the contributing guidelines.
• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective or that my feature works.
• New and existing unit tests pass locally with my changes.
• I have updated the documentation accordingly.