[Feature]: Add integration scaffolder for contributors

[PascalThuet]

Summary

Adding a new built-in integration still requires hand-editing several files and remembering naming rules. That is slow and easy to get wrong.

Proposal

Add a contributor-facing scaffold command:

specify dev integration scaffold <key> --type markdown
specify dev integration scaffold <key> --type toml
specify dev integration scaffold <key> --type yaml
specify dev integration scaffold <key> --type skills

The command should generate a minimal integration package and test skeleton, then print the registry edits still required.

Acceptance Criteria

• Creates src/specify_cli/integrations/<package>/__init__.py.
• Creates tests/integrations/test_integration_<package>.py.
• Supports markdown, toml, yaml, and skills base classes.
• Converts hyphenated keys to Python-safe package/test names.
• Refuses to overwrite existing files unless an explicit force flag is added later.
• Prints concise next steps for registry registration and verification.