refactor(ai): rebranded openrouter identifiers and user agent to omp

- Updated user agent and openrouter titles in packages/ai from Oh-My-Pi to omp.
- Integrated getOpenRouterHeaders into image generation tool requests in packages/coding-agent.
- Updated provider documentation and test assertions to reflect the rebrand.

scripts/rewrite-system-prompt.ts

@@ -417,7 +417,7 @@ export function makeOpenRouterRewriter(opts: OpenRouterOptions): RewriteChunk {
 						Authorization: `Bearer ${opts.apiKey}`,
 						"Content-Type": "application/json",
 						"HTTP-Referer": "https://omp.sh/",
-						"X-Title": "Oh-My-Pi",
+						"X-Title": "omp",
 					},
 					body,
 				});