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>

server/content/content.yaml

@@ -0,0 +1,11 @@
+# Content manifest — The Settled Reach v0.1
+# The server reads this first to discover campaigns and content layout.
+# District discovery uses glob patterns — no per-district listing needed.
+version: "0.1.0"
+
+campaigns:
+  - id: "main"
+    path: "campaigns/main"
+    enabled: true
+    discovery:
+      districts: "systems/**/districts/*/district.yaml"