feat(init): implement interactive wizard for agk init -i

[kunalkushwaha]

What

Implements the interactive init wizard. The -i/--interactive flag was already parsed and passed into GenerateOptions.Interactive, but no generator ever read it — so agk init -i behaved exactly like a non-interactive init. This wires it to a real guided setup flow.

This is feature A1 from the FEATURES.md roadmap (and the long-standing "Interactive Init Wizard" roadmap item).

Behavior

$ agk init -i
🧙 AGK interactive project setup

Project name [my-agent]: wizbot
Template:
  * 1) quickstart
    2) workflow
Choose (number or name) [quickstart]: 2
LLM provider:
  * 1) openai
    2) anthropic
    3) ollama
Choose (number or name) [openai]: ollama
Description (optional): My wizard agent

📦 Creating new AgenticGoKit project: wizbot ...

• Prompts for project name, template, LLM provider, and description, then scaffolds the chosen project.
• agk init -i can be run with no project-name arg — the wizard asks for it (args validation relaxed for interactive mode).
• Choices accept a number or a name; empty input takes the default; EOF falls back to defaults (no hang).

Design for testability

The prompt flow is a small prompter decoupled from os.Stdin/os.Stdout (io.Reader/io.Writer), so the wizard is unit-tested with piped input.

Testing

• go build, go vet, go test ./..., gofmt all green.
• Unit tests: full wizard flow, name prompting, EOF-defaults, invalid-then-valid choice.
• Verified end-to-end by piping answers into agk init -i: scaffolds the workflow template with the ollama provider correctly wired into the generated main.go (plugins/llm/ollama, Provider: "ollama").

Independent branch off main (alongside #13–#17).

🤖 Generated with Claude Code