feat: Add depth control flag to limit context generation scope

[BETAER-08]

Problem

Currently, amdb generate fetches the entire dependency graph relative to the focus file.
For complex projects, this creates an excessively large context file filled with distant dependencies, which adds noise when uploaded to AI tools.

Proposed Solution

• Add a --depth argument to the generate command (default: 1).
  • 0: Focus file only.
  • 1: Focus file + immediate dependencies.
  • 2: Focus file + nested dependencies.
• Modify ContextGenerator in src/core/generator.rs to stop graph traversal at the specified depth.

Definition of Done

• amdb generate --focus main.rs --depth 0 outputs only main.rs.
• Users can control the granularity of the generated context.