Craft npm target OIDC detection via CI environment variables

The isOidcEnvironment() helper in src/targets/npm.ts detects OIDC capability by checking CI-specific env vars that npm itself uses for OIDC token exchange: - GitHub Actions: ACTIONS_ID_TOKEN_REQUEST_URL AND ACTIONS_ID_TOKEN_REQUEST_TOKEN (both present when id-token: write permission is set) - GitLab CI/CD: NPM_ID_TOKEN (present when id_tokens with aud: "npm:registry.npmjs.org" is configured) This auto-detection means zero config changes for the common case. The explicit oidc: true config is only needed to force OIDC when NPM_TOKEN is also set (e.g., migration period).