perf: Optimize Call Graph Traversal Algorithm for Deep Context Generation

[BETAER-08]

Problem

The --depth context expansion logic in ContextGenerator::generate currently loads all relationship edges from the database into a Vec and performs a full-scan loop for every depth iteration. For large projects with thousands of nodes and edges, this $O(E \times \text{Depth})$ operation becomes exponentially slow.

Proposed Solution

• Optimize the graph traversal logic in src/core/generator.rs.
• Option A: Transform the edge list into an Adjacency List (HashMap<String, Vec<String>>) in memory so that neighbor lookups take $O(1)$ time.
• Option B: Utilize SQLite's Recursive CTE in src/db/query.rs to delegate the deep traversal entirely to the database engine.

Definition of Done

• Graph traversal logic is updated (Adjacency List or Recursive CTE).
• Generation time with --depth 2 or higher is significantly reduced on large repositories.
• The output context remains exactly the same as the unoptimized version.