Add explain command and MCP explain_structure tool

[httpdss]

Motivation

• Provide a safe preview mode to inspect how a structure definition will resolve (files, folders, nested structures, remote references, variables and hooks) without creating files or running hooks.
• Make that preview available to MCP clients so remote tooling can request explanations programmatically.
• Improve developer UX by reporting conflict behaviour (skip, skip_if_exists, etc.) and resolved template variables prior to generation.

Description

• Add a new CLI command explain implemented in structkit/commands/explain.py that computes a non-destructive explanation of a structure and can output text or JSON.
• Integrate the command into the top-level parser via structkit/main.py and document it in docs/cli-reference.md.
• Add MCP support by registering an explain_structure tool and exposing _explain_structure_logic and a compatibility handler in structkit/mcp_server.py, allowing MCP clients to request text or JSON explanations.
• Add tests in tests/test_explain_command.py and update MCP advertising in structkit/commands/mcp.py; the explain implementation reports remote file refs, nested structures, hooks, resolved variables, and conflict actions.

Testing

• Ran pytest -q tests/test_explain_command.py and the explain-specific tests passed.
• Ran pytest -q tests/test_mcp_integration.py tests/test_commands.py tests/test_commands_more.py and those suites passed.
• Ran the full test suite with pytest -q and all tests passed (no failures).

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Codex Task