[templates] make scaffolding output deterministic with stable file or…

[aishagbadamosi]

…dering

Sort directory entries alphabetically in copy_template_contents() and switch TemplateContext::sections from HashMap to BTreeMap so that identical inputs always produce byte-identical output regardless of platform or run.

• collect read_dir entries into a Vec and sort by file name before processing
• replace HashMap with BTreeMap for deterministic section iteration
• sort list_custom_templates() output
• add determinism tests: reproducibility, sorted order, placeholder correctness, error on missing source, error on read-only destination
• document determinism guarantees in TEMPLATE_MARKETPLACE.md

Closes #685

Description

Sort directory entries alphabetically in copy_template_contents() and switch
TemplateContext::sections from HashMap to BTreeMap so that identical inputs
always produce byte-identical output regardless of platform or run.

• collect read_dir entries into a Vec and sort by file name before processing
• replace HashMap with BTreeMap for deterministic section iteration
• sort list_custom_templates() output
• add determinism tests: reproducibility, sorted order, placeholder correctness,
  error on missing source, error on read-only destination
• document determinism guarantees in TEMPLATE_MARKETPLACE.md

Closes #685

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Changes Made

• Change 1
• Change 2
• Change 3

Testing

How has this been tested?

Describe the tests you ran and how to reproduce them.

• Unit tests added/updated
• Integration tests added/updated
• Manual testing performed

Test Coverage

Describe what scenarios have been tested:

• Happy path:
• Edge cases:
• Error handling:

Code Quality Checklist

• My code follows the style guidelines of this project (cargo fmt)
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings (cargo clippy -- -D warnings)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• The CI checks pass (format, clippy, tests)

Breaking Changes

• This PR introduces breaking changes

If checked, describe the breaking changes and migration path:

Documentation

• README.md updated
• DEVELOPER_GUIDE.md updated (if applicable)
• API_REFERENCE.md updated (if applicable)
• No documentation changes needed

Screenshots (if applicable)

Add screenshots or GIFs for UI changes.

Additional Context

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Note: Make sure all tests pass locally before submitting:

cargo test
cargo fmt --all
cargo clippy -- -D warnings

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