feat(cli): add callers, callees, impact commands for CLI/MCP parity (#204)

Add `codegraph callers`, `codegraph callees`, and `codegraph impact` CLI commands, bringing the CLI to parity with the codegraph_callers/callees/impact MCP tools — so the graph-traversal queries work in scripts, CI, and git hooks without a running MCP server. All three support `--path` and `--json`; `impact` groups output by file to match the MCP layout.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>