… information

- Add detailed error information to the query failure error message
- Include source and error details for each failed document in the error message
- Format error details as "source (error)" pairs joined by semicolons

Add support for optional reasoning index in document retrieval pipeline.
The reasoning index enables fast-path lookups for specific query patterns
like document summaries and overview questions. Update the Engine and
Retriever interfaces to pass reasoning index through the query flow.

refactor(pilot): update guide_start method signature

Change the guide_start method in Pilot trait to accept a start_node
parameter, allowing pilots to evaluate the correct children instead of
always using root's children. Update LlmPilot and NoopPilot implementations
accordingly.

feat(toc-navigator): implement keyword overlap fallback for abstract queries

Add Phase C to ToCNavigator that uses keyword overlap matching against
section summaries when BM25 produces no results. This handles abstract
queries like "What is this project about?" where query terms don't appear
in section titles but summaries contain relevant matches.

fix(evaluate-stage): use BFS for leaf content collection

Switch from DFS to BFS (FIFO) traversal in collect_leaf_content to preserve
document order - ensuring the first section in the document appears first
in the output.

feat(evaluate-stage): detect stagnant candidate loops

Implement stagnant candidate detection in EvaluateStage to identify when
candidates remain unchanged between iterations, indicating that further
backtracking won't improve results.

refactor(search-stage): enhance summary query detection

Improve summary query detection logic to handle phrase patterns with
filler word removal, making it more effective at identifying overview
requests like "what is this project about".

refactor(persistence): fix serialization drift issues

Change PersistedWrapper to store payload as serde_json::Value to avoid
HashMap key ordering drift during repeated serialization/deserialization
cycles.

…ndling

- Change return type from `Result<(), Box<dyn std::error::Error>>` to
  `vectorless::Result<()>`
- Rename client variable to engine for consistency
- Update workspace path to "./worksspace_flow_example"
- Fix typo in API key format from "sk-..." to "sk..."
- Add proper endpoint configuration with "https://api"
- Simplify error mapping by removing explicit type annotation
- Update query text to "What is the seconds for complex multi-hop?"
- Fix content preview truncation logic
- Comment out document removal step for cleanup