Make agentic azd init more discoverable so that I can discover and try experimental AI features without needing to know about hidden alpha flags.

Background
Currently, the LLM/AI agent features are hidden behind the alpha.llm configuration flag. To use the feature, users must:

• discover the flag exists in azd main menu
• run azd config set alpha.llm on
• then run azd init to use AI-assisted initialization

This creates a discoverability problem—users who would benefit from these features don't know they exist.

Proposed solution
Add option in the init flow to discover agentic init. When a customer runs azd init and the alpha feature isn't enabled, prompt them to enable it. This surfaces the feature and maintains opt-in behavior for alpha experimental features.

Acceptance Criteria

• azd init shows option for Use agent mode (experimental)
• When selected without alpha.llm enabled, display clear instructions: azd config set alpha.llm on "To use this feature run azd config set alpha.llm on and then run azd init again"
• When selected with alpha.llm already enabled, proceed directly to agent mode