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-checklist

@@ -16,8 +16,8 @@ import jsonschema
 import yaml
 
 ROOT = Path(__file__).resolve().parent.parent
-SCHEMA_PATH = ROOT / "content" / "_schema" / "checklist.schema.json"
-GAUNTLET_DIR = ROOT / "content" / "gauntlet"
+SCHEMA_PATH = ROOT / "server" / "content" / "_schema" / "checklist.schema.json"
+GAUNTLET_DIR = ROOT / "server" / "content" / "gauntlet"
 
 
 def load_schema():