Improve list_indices to discover indices via _settings pattern fallback

[kapral18]

Problem

Currently, list_indices only discovers indices by looking for aliases ending with -repo. However, the semantic-code-search-indexer doesn't automatically create such aliases when indexing a repository. This creates a workflow issue where users must manually:

1. Create an index with <index>-repo name
2. Add a <index>-repo alias

This is a design omission that makes the tool less user-friendly.

Solution

Implement a dual discovery strategy:

1. Primary: Continue to discover indices via aliases ending with -repo (backward compatible)
2. Fallback: Discover indices by finding all indices ending with _settings, extracting the base name, and verifying the base index exists

The indexer creates both <name> and <name>_settings indices, so we can identify repos by the _settings pattern.

Implementation

• Add discoverRepoIndicesFromAliases() function (primary strategy)
• Add discoverRepoIndicesFromSettings() function (fallback strategy)
• Merge and deduplicate results (aliases take precedence for display names)
• Update both listIndices() and getAvailableIndices() to use the same discovery logic
• Add comprehensive test coverage including console.warn verification

Benefits

• Works immediately with current indexer behavior (no manual alias creation needed)
• Maintains backward compatibility with existing -repo aliases
• Provides clear migration path for future indexer improvements
• Future-proof: works with both old and new indexer versions

[kapral18]

Related to elastic/semantic-code-search-indexer#130 - The indexer will now automatically create -repo aliases, making this MCP server improvement even more effective!

[kapral18]

Closing in favor of alias-first architecture. See new issue #XXX for details.

[kapral18]

Closing in favor of alias-first architecture. See #35 for details.