docs: add platform controls page for Coder Agents (#22680)

Adds a docs page under /docs/ai-coder/agents describing our philosophy
on platform team control over agent behavior: admin-level configuration,
zero developer options, enforcement over defaults. Covers what's
available today (providers, models, system prompt, template routing) and
where we're headed (usage analytics, infra-level enforcement, tool
customization).

docs/manifest.json

@@ -1201,6 +1201,11 @@
 							"title": "Models",
 							"description": "Configure LLM providers and models for Coder Agents",
 							"path": "./ai-coder/agents/models.md"
+						},
+						{
+							"title": "Platform Controls",
+							"description": "How platform teams control agent behavior, models, and policies",
+							"path": "./ai-coder/agents/platform-controls.md"
 						}
 					]
 				}