feat(P21): environments concept + onboarding scaffold (REQ-61)

---ci---
project: acdl
phase: 21
milestone: v1.6
status: execute
---/ci---

Introduce platform-managed environments: a consumer does not provide an
AWS account, VPC, subnet, S3 state bucket, or runner key. A named
environment is a platform-owned bundle of account + network + state
backend + IAM role (surfaced via ABAC), selected by name in the contract.

Scaffold:
- core/environments/dev.json (sample dev env definition)
- core/environments/README.md (how envs are used + how to add one)
- core/environment_check.py (reads a contract's environment field,
  looks up core/environments/<name>.json, prints a friendly onboarding
  prompt when none exists, exits non-zero)
- scripts/run_platform.sh: Step 0 calls environment_check.py before
  contract validation; a missing env halts the pipeline with the
  onboarding prompt
- tests/test_environment_check.py: 12 tests (dev bound, missing env ->
  onboarding prompt, onboarding message lists provisions, contract
  paths, wire-in, check-only still passes)

Tests: 166 pass (154 + 12 new).

tests/test_environment_check.py

@@ -0,0 +1,99 @@
+import sys
+from pathlib import Path
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+ROOT = Path(__file__).resolve().parent.parent
+
+from core.environment_check import check, _onboarding_message
+
+
+class TestEnvironmentCheck:
+    def test_dev_environment_is_bound(self):
+        ok, msg = check(env_name="dev", root=ROOT)
+        assert ok is True
+        assert "dev" in msg
+
+    def test_missing_environment_emits_onboarding_prompt(self):
+        ok, msg = check(env_name="nonexistent-env", root=ROOT)
+        assert ok is False
+        assert "nonexistent-env" in msg
+        assert "onboarding" in msg.lower() or "Environment Onboarding" in msg
+        assert "platform team" in msg.lower()
+
+    def test_onboarding_message_lists_platform_provisions(self):
+        msg = _onboarding_message("qa")
+        assert "qa" in msg
+        assert "AWS account" in msg
+        assert "network" in msg.lower()
+        assert "state backend" in msg.lower()
+        assert "IAM role" in msg
+
+    def test_contract_with_dev_environment_passes(self):
+        ok, msg = check(contract_path=str(ROOT / "contracts/static-asset.yaml"), root=ROOT)
+        assert ok is True
+        assert "dev" in msg
+
+    def test_contract_with_missing_environment_fails(self, tmp_path):
+        contract = tmp_path / "contract.yaml"
+        contract.write_text(
+            "uses: acdl/pipelines/deploy.yaml@v1.4\n"
+            "module: static-asset\n"
+            "environment: no-such-env\n"
+            "inputs:\n  bucket_name: x\n  region: us-east-1\n"
+        )
+        ok, msg = check(contract_path=str(contract), root=ROOT)
+        assert ok is False
+        assert "no-such-env" in msg
+
+    def test_no_contract_or_env_returns_false(self):
+        ok, msg = check(root=ROOT)
+        assert ok is False
+
+    def test_contract_without_environment_field_returns_false(self, tmp_path):
+        contract = tmp_path / "contract.yaml"
+        contract.write_text(
+            "uses: acdl/pipelines/deploy.yaml@v1.4\n"
+            "module: static-asset\n"
+            "inputs:\n  bucket_name: x\n  region: us-east-1\n"
+        )
+        ok, msg = check(contract_path=str(contract), root=ROOT)
+        assert ok is False
+
+
+class TestEnvironmentDefinitions:
+    def test_dev_environment_file_exists(self):
+        assert (ROOT / "core/environments/dev.json").is_file()
+
+    def test_dev_environment_file_is_valid_json(self):
+        import json
+        d = json.load(open(ROOT / "core/environments/dev.json"))
+        assert d["name"] == "dev"
+        assert "account_id" in d
+        assert "region" in d
+        assert "state_backend" in d
+        assert "network" in d
+        assert "runner_role_arn" in d
+
+    def test_environments_readme_exists(self):
+        assert (ROOT / "core/environments/README.md").is_file()
+
+
+class TestRunPlatformWireIn:
+    def test_run_platform_sh_calls_environment_check(self):
+        content = open(ROOT / "scripts/run_platform.sh").read()
+        assert "environment_check.py" in content
+        assert "Step 0: environment onboarding check" in content
+
+    def test_check_only_passes_with_dev_environment(self):
+        import subprocess
+        result = subprocess.run(
+            ["bash", str(ROOT / "scripts/run_platform.sh"), "--check-only"],
+            capture_output=True, text=True, cwd=str(ROOT),
+            timeout=30,
+        )
+        assert result.returncode == 0, f"stdout: {result.stdout}\nstderr: {result.stderr}"
+        assert "PLATFORM CHECK OK" in result.stdout
+        assert "environment" in result.stdout.lower() or "Step 0" in result.stdout