fix: resolve data races in deferred tools and BM25 strategy, prevent TUI editor panic

[Piyush0049]

Summary

This PR addresses two critical concurrency safety bugs and one runtime panic condition in the codebase.

Changes Included

1. Deferred Tools Mutex Fix
   • File: pkg/tools/builtin/deferred/deferred.go
   • Fixed a memory safety bug where ToolSet.Start mutated the deferredTools map while holding only a read lock. Upgraded this to a write lock.
2. RAG BM25 Strategy Concurrency Protection
   • File: pkg/rag/strategy/bm25.go
   • Added fileHashesMu sync.Mutex to guard concurrent reads and writes to the fileHashes map from background watcher goroutines.
3. External Editor Slice Panic Guard
   • File: pkg/tui/tui.go
   • Guarded the parser against whitespace-only environment variables which caused index out of bounds panics.

Validation

• All unit tests in the modified packages passed.
• Linting ran cleanly with zero issues.