Add sources command for custom structures

[httpdss]

Motivation

• Provide a way to save, validate and reuse local custom structure directories as named sources to simplify reuse and automation.
• Allow selecting custom structure sources via a --source flag or a source-prefixed structure name (e.g. company/project/python) while preserving existing --structures-path / STRUCTKIT_STRUCTURES_PATH behavior.

Description

• Add structkit/sources.py with utilities for reading/writing a user-level sources.json, validation helpers, source-prefix parsing, resolve_structures_path, and SourceConfigError for error handling.
• Add structkit/commands/sources.py implementing the structkit sources command group with list, add, remove, show, and validate subcommands and register it in the CLI via structkit/main.py.
• Integrate --source and source-prefix resolution into generate, info, list, and search by calling resolve_structures_path and handling SourceConfigError to preserve precedence: CLI --structures-path / env var > --source > source-prefixed names > bundled contribs.
• Document named sources in docs/custom-structures.md and add README examples; include new tests in tests/test_sources_command.py.

Testing

• Ran pytest tests/test_sources_command.py and the targeted tests which passed.
• Ran the full test suite with pytest and all tests passed (164 passed).
• Executed a quick CLI flow (adding a source, listing it and resolving an info/list against a named source) in an automated script which exercised the new commands and resolution logic as part of automated checks.

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Codex Task