refactor(env): migrated environment variables from OMP_ to PI_ prefix and centralized access via getEnv()

- Migrated environment variable access from direct process.env to centralized getEnv() utility function across all packages.
- Renamed environment variable prefix from OMP_ to PI_ throughout codebase (e.g., OMP_CODING_AGENT_DIR -> PI_CODING_AGENT_DIR).
- Removed automatic environment variable migration from PI_ to OMP_ prefixes via migrate-env.ts module.
- Removed env setting from configuration schema and applyEnvironmentVariables() method from settings.
- Updated CI/CD build configuration to use PI_COMPILED flag instead of OMP_COMPILED.
- Changed venvPath property in PythonRuntime from nullable (string | null) to optional (string | undefined).

packages/coding-agent/src/task/omp-command.ts

@@ -1,5 +1,7 @@
 import process from "node:process";
 
+import { getEnv } from "@oh-my-pi/pi-utils";
+
 interface OmpCommand {
 	cmd: string;
 	args: string[];
@@ -10,7 +12,7 @@ const DEFAULT_CMD = process.platform === "win32" ? "omp.cmd" : "omp";
 const DEFAULT_SHELL = process.platform === "win32";
 
 export function resolveOmpCommand(): OmpCommand {
-	const envCmd = process.env.OMP_SUBPROCESS_CMD;
+	const envCmd = getEnv("PI_SUBPROCESS_CMD");
 	if (envCmd?.trim()) {
 		return { cmd: envCmd, args: [], shell: DEFAULT_SHELL };
 	}