Show diffs when sync overwrite replaces files

[coisa]

Objective

Show a diff in sync summaries when overwrite would replace consumer files, instead of silently relying on backup files.

Current Limitation

When --overwrite replaces an existing consumer file, maintainers need to understand exactly what changed. Backup files can reduce data loss, but a clear diff in the sync summary is more useful for review and CI logs.

Proposed Work

Enhance overwrite reporting so dev-tools:sync displays a readable diff for overwritten files. Prefer using an existing PHP diff library already available through PHPUnit or its dependencies, when suitable, instead of implementing a custom diff engine.

Scope

• Show before/after diffs for overwritten file resources.
• Include the path being overwritten and the source resource.
• Integrate with the final sync summary when available.
• Keep output readable in terminal and CI logs.
• Avoid writing backup files as the primary strategy unless a separate explicit option is introduced later.

Non-goals

• Creating backup files by default.
• Implementing an interactive prompt.
• Showing diffs for binary files in the first implementation.

Acceptance Criteria

Functional Criteria

• dev-tools:sync --overwrite reports a diff for each text file it replaces.
• The diff appears in the sync output or final summary with the affected path.
• Unchanged overwritten targets are reported as no-op or unchanged.
• Binary or unreadable files are handled gracefully with a clear message.
• Tests cover changed text files, unchanged files, and non-text/binary fallback behavior.

Architectural / Isolation Criteria

• Diff rendering is isolated behind a small service/helper.
• The implementation reuses a maintained diff component when practical.
• Output is deterministic enough for assertions.