feat(environment): introduce disable_omo_env configuration option

- Added a new configuration option `disable_omo_env` to control the injection of the `<omo-env>` block in agent prompts.
- Updated relevant functions and tests to support this feature, ensuring that the environment context can be toggled on or off as needed.
- Enhanced documentation to reflect the new option and its implications for API cost and cache hit rates.

src/agents/builtin-agents/environment-context.ts

@@ -1,8 +1,16 @@
 import type { AgentConfig } from "@opencode-ai/sdk"
 import { createEnvContext } from "../env-context"
 
-export function applyEnvironmentContext(config: AgentConfig, directory?: string): AgentConfig {
-  if (!directory || !config.prompt) return config
+type ApplyEnvironmentContextOptions = {
+  disableOmoEnv?: boolean
+}
+
+export function applyEnvironmentContext(
+  config: AgentConfig,
+  directory?: string,
+  options: ApplyEnvironmentContextOptions = {}
+): AgentConfig {
+  if (options.disableOmoEnv || !directory || !config.prompt) return config
   const envContext = createEnvContext()
   return { ...config, prompt: config.prompt + envContext }
 }