feat: add buzz create command for interactive goal creation

[Copilot]

Adds a new buzz create command that interactively prompts users for goal parameters. Previously, users could only create goals through the TUI modal (press 'n') or manually via the API.

Changes

• Command handler: handleCreateCommand() prompts for slug, title, type, units, and 2 of 3 parameters (goaldate/goalval/rate)
• Input helper: promptUser() reads stdin with scanner error handling
• Validation: Reuses existing validateCreateGoalInput() from TUI modal
• Integration: Updated help text and error messages

Example

$ buzz create
Create a new Beeminder goal
===========================

Goal slug: reading
Goal title: Daily Reading
Goal type (default: hustler): 
Goal units: pages
Goal date: null
Goal value: 365
Rate: 1

Creating goal...
Successfully created goal: reading

The command follows the same patterns as buzz add and buzz charge - checks authentication, validates inputs, calls existing API functions, and displays results.

Original prompt

Let's create a new command for creating new Beeminder goals. It should ask the user for details interactively (slug, rate, etc)

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