[ENHANCEMENT] Add Dart support to codebase indexing #940

Problem (one or two sentences)

Zoo Code does not currently support Dart when indexing a codebase. As a result, codebase indexing cannot provide useful context for Dart projects

Context (who is affected and when)

This affects Dart and Flutter developers who use Zoo Code in Dart repositories. I work primarily with Dart projects, so the codebase indexing feature is not useful for most of my work

Desired behavior (conceptual, not technical)

Zoo Code should support indexing Dart source code so that it can understand and retrieve relevant context from Dart and Flutter projects

Constraints / preferences (optional)

Dart indexing should work consistently with the other supported languages and handle common Dart and Flutter project structures

Request checklist

• I've searched existing Issues and Discussions for duplicates
• This describes a specific problem with clear context and impact

Zoo Code Task Links (optional)

#751

Acceptance criteria (optional)

• Dart source files are included in codebase indexing.
• Relevant Dart code can be found through codebase search.
• Common Dart and Flutter project structures are supported.
• Adding Dart support does not negatively affect indexing for existing languages.

Proposed approach (optional)

No response

Trade-offs / risks (optional)

No response