feat: add blast-radius / impact analysis (what breaks if I change symbol X)

[aider4ryder]

Motivation

codeseek currently provides codeseek_callers (direct callers of a symbol) and codeseek_callgraph (local call graph neighborhood). But it lacks transitive impact analysis — answering questions like:

• "If I change the return type of parse_config(), what downstream code breaks?"
• "If I delete DatabasePool::acquire(), what's the blast radius?"
• "What's the dependency path from handle_request() to execute_query()?"

These are the most valuable questions during refactoring, and right now users must manually chain codeseek_callers N times.

Proposed tools

codeseek_impact — transitive reverse reachability

codeseek_impact(symbol: str, max_depth: int = 5) → list of all symbols that transitively depend on `symbol`

codeseek_path — shortest call path between two symbols

codeseek_path(from: str, to: str) → list of edges showing the call chain

Implementation note

The call graph data already exists from tree-sitter parsing during indexing. These queries are pure graph traversal (BFS/DFS) on the existing edges — no additional model calls needed. The LanceDB index could store edges in a separate table for efficient graph queries.

This would make codeseek a complete replacement for tools like CodeGraph/GitNexus, which already provide this functionality.