Refactor CLI build handling into helper functions

[leynos]

Summary

• extract build orchestration into a dedicated handle_build helper
• factor out manifest path resolution and Ninja file creation helpers
• keep CLI run shallow for easier maintenance

Testing

• cargo fmt --all
• make lint
• make test
• make fmt (fails: docs/srgn.md:99 MD040/fenced-code-language Fenced code blocks should have a language specified)

closes #47

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https://chatgpt.com/codex/tasks/task_e_6895d788cd8883229d612eac1cd7e13c

Summary by Sourcery

Refactor the CLI’s build handling by extracting the build orchestration and file operations into dedicated helper functions to simplify the run dispatcher and improve maintainability.

Enhancements:

• Extract the Build command logic into a new handle_build helper.
• Introduce write_ninja_file, create_temp_ninja_file, and resolve_manifest_path helpers to centralize file creation and manifest resolution.
• Simplify the run function by delegating Build and Manifest actions to the new helpers.
• Refactor generate_ninja to use resolve_manifest_path for manifest discovery.

Documentation:

• Add documentation and examples for the new helper functions.