Add stellar scaffold clean command to remove build artifacts

[Copilot]

Implements stellar scaffold clean subcommand to remove scaffold-generated artifacts from target/stellar, packages/*, and src/contracts/*.

Changes

• New clean subcommand (src/commands/clean.rs)

  • Removes target/stellar directory entirely
  • Cleans packages/* except git-tracked files and .gitkeep
  • Cleans src/contracts/* except git-tracked files and util.ts
  • Falls back to preserving common template files when git unavailable

• Command registration (src/commands/mod.rs)

  • Wired into CLI with proper error handling

• Test coverage (tests/it/unit.rs)

  • Verifies artifact removal and file preservation

Usage

stellar scaffold clean
stellar scaffold clean --manifest-path path/to/Cargo.toml

Output Example

🧹 Cleaning scaffold artifacts from /path/to/project
  Removing target/stellar/
  Cleaning packages/ (preserving git-tracked files)
    Removed packages/my-package
  Cleaning src/contracts/ (preserving git-tracked files)
    Removed src/contracts/generated.ts
✨ Clean complete!

Original prompt

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This section details on the original issue you should resolve

<issue_title>stellar scaffold reset to clear all artifacts</issue_title>
<issue_description>Add stellar scaffold reset command to clear all scaffold-generated artifacts from:

• target/stellar
• contract aliases (used to be able to just rm .stellar directory, but with only-global config this no longer works; we will need to find a different way)
• packages/* (but not checked-into-git files like .gitkeep)
• src/contracts/* (but not checked-into-git files like util.ts)</issue_description>

<agent_instructions>Create a new sub command in stellar-scaffold-cli called clean. Ignore the issues around contract aliases. Also add a test.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@willemneal I prefer `clean`. @tupui > I prefer `clean`.

cargo clean I guess ;) But git reset. I get both are fine. </comment_new>
<comment_new>@pselle
What if this were written out as a small design proposal?

Problem Statement

Proposed Solution

Alternative solutions & Rejected ideas</comment_new>

• Fixes stellar scaffold reset to clear all artifacts #259

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