Add JSON GraphRenderer + !graph --format flag

[jogrogan]

Summary

Adds a second GraphRenderer implementation — JsonGraphRenderer — alongside the
existing Mermaid renderer, and exposes renderer selection through the !graph CLI
command via a new --format flag. This gives programmatic consumers a structured,
parse-friendly view of the same pipeline topology the Mermaid renderer draws, without
scraping Mermaid text.

Builds on the pluggable graph SPI introduced in #222. No changes to the graph data
model (PipelineGraph / GraphNode / GraphEdge) or providers — this is purely a new
renderer plus CLI plumbing.

JSON is more easily sent over the wire and rendered by web UIs.

What's included

hoptimator-graph

• New JsonGraphRenderer (format() == "json") that serializes a PipelineGraph to a
  stable JSON document. Registered via META-INF/services so it's discovered through
  GraphService exactly like the Mermaid renderer.
• Adds a jackson-databind dependency (used for correct escaping / tree building).

hoptimator-cli

• !graph <identifier> [--depth N] [--format mermaid|json]. Default remains
  mermaid — existing behavior and integration scripts are unchanged. --format
  dispatches through GraphService.render(graph, format) to the matching SPI.
• Unsupported formats are rejected with the list of available formats (validated against
  GraphService.availableFormats()).
• The degenerate-resource warning (a Mermaid %% comment) is now only emitted for
  Mermaid output, so JSON output stays valid/parseable.

Docs

• docs/user-guide/sql-cli.md: documents --format with a JSON example.
• docs/extending/index.md: notes that both Mermaid and JSON renderers now ship.

JSON format

{ "root": "<GraphNode.id of the focal node>", "orientation": "LR" | "TD",          // TD when the root is a LogicalTable, else LR "nodes": [
   { "id", "kind", "displayName", ...kind-specific fields },
   // PIPELINE:       name, jobKind?, engine?, executionMode?
   // VIEW:           name, materialized
   // LOGICAL_TABLE:  name, tiers { tier -> database }
   // TRIGGER:        name, schedule?, paused, jobTemplateName?, containerName?
   // EXTERNAL:       database, path[] ], "edges": [ { "from", "to", "type" } ]  // includes OWNER_OF
}

[github-actions]

Code Coverage

Overall Project	84.49% -0.04%	🟢
Files changed	94.97%	🟢

File	Coverage
JsonGraphRenderer.java	94.97% -5.03%	🟢