Add `structkit sources` command to manage custom structure sources

[httpdss]

Summary

Add a structkit sources command group for managing named custom structure sources.

Motivation

StructKit currently supports custom structures through STRUCTKIT_STRUCTURES_PATH and command flags. A source-management command would make custom template locations easier to configure, inspect, share, and use consistently.

Proposed behavior

Add a command group with subcommands such as:

• structkit sources list
• structkit sources add <name> <path-or-url>
• structkit sources remove <name>
• structkit sources show <name>
• structkit sources validate <name>

Sources could initially support local filesystem paths, with remote source support considered later.

Configuration considerations

• Store source configuration in a user-level StructKit config file.
• Preserve compatibility with STRUCTKIT_STRUCTURES_PATH.
• Define clear precedence when both named sources and --structures-path are used.
• Avoid changing existing generate, list, search, and info behavior unless a source is explicitly selected.

Example usage

• structkit sources add company ./templates
• structkit sources list
• structkit list --source company
• structkit generate company/project/python ./app

Acceptance criteria

• Command group is registered in the CLI.
• Users can add, list, show, validate, and remove local sources.
• Existing structures-path behavior remains compatible.
• Tests cover source config read/write, invalid paths, missing sources, and precedence behavior.
• Documentation explains how sources relate to STRUCTKIT_STRUCTURES_PATH.