Add LSP shim for cdidx

[Widthdom]

Summary

• Add cdidx lsp as a read-only stdio Language Server Protocol shim backed by the existing CodeIndex database.
• Expose initialize, workspace symbols, document symbols, definitions, and references for LSP-native editors.
• Document the CLI/MCP/LSP integration surface and add the changelog fragment for No Language Server Protocol (LSP) shim; MCP-only locks out LSP-native editors #1650.

Validation

• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -p:UseSharedCompilation=false --filter LspServerTests
• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll status --check --json
• Codex adversarial review: No blocking/actionable issues found.

Fixes #1650