Add named structure sources management (CLI + MCP)

[httpdss]

Motivation

• Provide a convenient way to register and reuse named local structure sources so users don't have to pass --structures-path or set STRUCTKIT_STRUCTURES_PATH repeatedly.
• Allow structure resolution to accept a <source>/<structure> prefix and a --source flag, while preserving --structures-path precedence.
• Expose source management over MCP so external tools can list/add/remove/validate named sources programmatically.

Description

• Add new utilities in structkit/sources.py including read_sources, write_sources, add_source, remove_source, resolve_structures_path, and related helpers and SourceError for config handling and validation.
• Add a new CLI command group structkit sources in structkit/commands/sources.py with subcommands list, add, remove, show, and validate and support for --config-path (STRUCTKIT_SOURCES_CONFIG).
• Integrate named source resolution into generate and list via a new --source option and by calling resolve_structures_path, and register SourcesCommand in structkit/main.py.
• Extend MCP server (structkit/mcp_server.py) with manage_sources tool and wire generate_structure to accept an optional source parameter so MCP generation resolves named sources.
• Update documentation in docs/custom-structures.md and docs/cli-reference.md to describe sources usage and resolution precedence, and add tests/test_sources_command.py covering config read/write, CLI flows, precedence, and MCP management.

Testing

• Ran the full test suite with pytest which completed successfully: 185 passed.
• Ran the new focused tests with python -m pytest tests/test_sources_command.py which completed successfully: 7 passed.
• Verified module compilation with python -m py_compile for the modified modules and inspected structkit sources --help output; all checks succeeded.

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Codex Task