feat: Enhance custom template generation

[c12i]

This PR replaces the separate custom-template with a system that builds custom templates from existing template bases (vue, svelte, lit, react, headless).

Changes

• Removed standalone custom-template
• Added ability to use existing templates as bases for custom templates
• Shifted from nix flake init to CLI-based template generation

Benefits

• Greater flexibility: Developers can now choose from multiple starting points (vue, svelte, lit, react, headless)
• Reduced maintenance: Eliminates need to maintain a separate custom template
• Less code duplication: Leverages existing template code

Trade-offs

• Deprecates nix flake init -t github:holochain/scaffolding workflow
• Requires using CLI directly to generate custom template base i.e nix run github:holochain/scaffolding#hc-scaffold -- template new or hc-scaffold template new if installed via cargo

Rationale

The increased flexibility and reduced maintenance overhead outweigh the loss of the nix flake init workflow, as developers gain access to multiple template starting points instead of being limited to the previous lit-based custom template.

TODOs

• Generate custom templates via CLI
• Update custom template documentation

Closes #388