Add explain command to preview structure resolution

[httpdss]

Motivation

• Provide a safe, no-side-effect way to inspect how a structure definition will resolve before generation, separate from generate --dry-run which simulates file operations and diffs.

Description

• Add a new CLI command explain implemented in structkit/commands/explain.py that resolves structure definitions and reports files, folders, nested folders[].struct traversal, declared variables and resolved values, hooks (rendered but not executed), remote file references (detected but not fetched), and configured conflict behavior.
• Register the command in the main CLI by adding ExplainCommand to get_parser() in structkit/main.py.
• Support both text and JSON output via --json/-o json, accept --vars, --mappings-file, and --file-strategy, and use the same Jinja2 rendering environment/filters for variable resolution and with rendering.
• Update documentation (docs/cli-reference.md, README.md) with explain usage, examples, and guidance on how it differs from generate --dry-run.
• Add unit tests in tests/test_explain_command.py to cover CLI registration, nested structures, remote references, rendered hooks/variables, conflict behavior for existing files, text and JSON outputs, and ensure no side effects (no fetches, no subprocess runs).

Testing

• Ran pytest tests/test_explain_command.py and the new tests passed.
• Ran the full test suite with pytest, resulting in all tests passing (158 passed).
• Performed static checks by compiling modules with python -m compileall structkit tests/test_explain_command.py which succeeded.
• Exercised the CLI manually with python -m structkit.main explain project/python --json to verify JSON output, which produced the expected explanation output.

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Codex Task