feat(generate): default structure_definition and base_path when omitted

[httpdss]

Summary
This PR improves the UX of the generate command by making both positional arguments optional with sensible defaults. Users can now run struct generate with no additional arguments when a .struct.yaml is present in the current directory.

Motivation

• Streamline the most common workflow: running generation from the current directory using a local .struct.yaml.
• Reduce friction and boilerplate for quick iterations and demos.

Changes

• CLI: Make positional args optional in GenerateCommand
  • structure_definition defaults to .struct.yaml
  • base_path defaults to .
  • Explicitly provided arguments continue to override these defaults.
• Docs: Updated to reflect new defaults
  • docs/cli-reference.md
  • docs/usage.md
  • docs/quickstart.md

Usage

# With defaults (if .struct.yaml exists in the current directory)
struct generate

# From a specific YAML file into a target directory (unchanged)
struct generate file://my-config.yaml ./output

# Using a built-in structure definition into a directory (unchanged)
struct generate python-basic ./my-project

Backward compatibility

• Existing commands remain valid; this is an additive improvement.
• Explicit arguments still take precedence over defaults.

Testing

• Full test suite executed locally: 112 tests passed.
• No behavior changes to existing flows other than allowing omitted positional args with sensible defaults.

Docs

• CLI reference updated to show optional positional args and defaults.
• Usage and Quickstart updated with examples using struct generate without arguments.

Closes #113.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.19%. Comparing base (3464fbf) to head (d44939e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #114      +/-   ##
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+ Coverage   68.06%   68.19%   +0.13%     
==========================================
  Files          20       20              
  Lines        1522     1522              
  Branches      269      269              
==========================================
+ Hits         1036     1038       +2     
+ Misses        410      409       -1     
+ Partials       76       75       -1     

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