feat: `cora callers` / `cora impact` — Call graph query commands

[ajianaz]

Description

Call graph navigation commands built on the symbol index.

cora callers AuthService.validate    # who calls this function?
cora impact   AuthService            # what breaks if I change this module?
cora impact --depth 3 src/api/       # deep dependency analysis

Why

Essential for understanding change impact — the #1 question before refactoring. CodeGraph has this as core feature. Cora can do it with the added benefit of LLM-assisted analysis.

Scope

• Call graph traversal using indexed symbol relationships
• Reverse dependency analysis (callers)
• Forward dependency analysis (callees / impact)
• Depth-limited traversal to control output size
• JSON output for programmatic use

Depends On

• feat: cora index — Symbol index & SQLite persistence #264 cora index
• feat: cora explore — Structural + semantic code search #265 cora explore

Strategy Ref

Code intelligence strategy #6 from competitive analysis.