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).

scripts/repro-stuck.ts

@@ -77,7 +77,7 @@ async function spawnBatch(count: number, basePort: number, timeout: number): Pro
 			stdout: "pipe",
 			stderr: "pipe",
 			stdin: "pipe",
-			env: { ...process.env, NO_COLOR: "1", OMP_DEBUG_STARTUP: "1" },
+			env: { ...process.env, NO_COLOR: "1", PI_DEBUG_STARTUP: "1" },
 		});
 		instances.push({ proc, port, stdout: "", stderr: "", status: "pending" });
 	}