Feature: Add codegraph export command

[Michael2150]

Add codegraph export command

Adds a new codegraph export CLI command that lets you dump your indexed graph to a file for visualization or further processing.

What's in this PR

• Four output formats: DOT (Graphviz), Mermaid flowchart, Cytoscape JSON, and a self-contained interactive HTML page (no server needed — just open it in a browser).
• Filtering options: --kind to pick specific node types, --edge-kind to filter relationship types, --all-kinds to include everything, and --limit to cap node count (defaults to 1000 with a warning when truncated).
• HTML viewer includes a sidebar with per-kind checkboxes, a live search, hover tooltips, and neighbourhood highlight on click — useful for quickly exploring a codebase's structure visually.
• Two new public API methods on CodeGraph: getAllNodes() and findEdgesBetweenNodes(), which the exporter uses internally but are also handy for anyone embedding the library.

Why

There was no way to get the graph data out of CodeGraph in a format you could hand to an external tool. DOT and Mermaid cover the "render it as a diagram" use case; Cytoscape JSON covers the "load it into my own viz tool" case; and the HTML format is the zero-friction option when you just want to poke around without setting anything up.

Usage

# Interactive HTML — open in browser
codegraph export -f html -o graph.html

# Graphviz
codegraph export -f dot | dot -Tsvg > graph.svg

# Only show functions and methods
codegraph export -f dot --kind function,method -o graph.dot

# Raise the node cap
codegraph export -f cytoscape --limit 5000 -o graph.json