refactor(simulation): move content/ under server/ (#669)

Content files (RON templates, YAML campaigns, gauntlet data) are consumed
exclusively by the server. Colocate them at server/content/ and update all
path references in Rust source, tooling scripts, and schema files.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tooling/validate-content

@@ -29,7 +29,7 @@ from pathlib import Path
 import jsonschema
 import yaml
 
-CONTENT_DIR = Path(__file__).resolve().parent.parent / "content"
+CONTENT_DIR = Path(__file__).resolve().parent.parent / "server" / "content"
 SCHEMA_DIR = CONTENT_DIR / "_schema"
 
 # Map directory parent name (or filename) to schema file