feat(P40): contract interpolation + environment JSON schema

---ci---
project: acdl
phase: 40
milestone: v1.9
status: execute
---/ci---

Phase 40 — contract-interpolation (REQ-103, REQ-104, D-081):

Interpolation:
- core/contract_resolver.py: _expand_vars(value, context) recursively
  expands ${env.<field>} + ${contract.<field>} tokens (dotted paths
  supported, e.g. ${env.state_backend.bucket}). Unknown tokens raise
  ValueError (fail loud). Expansion is post-schema-validation,
  pre-IR-resolution.
- resolve() accepts environment_override (D-088) — overrides the
  contract's environment field BEFORE schema validation so interpolation
  context is consistent.
- env context loaded via _load_env (self-contained, works as script +
  package import); 'environment' alias for env 'name' so
  ${env.environment} resolves.

Environment schema + bindings:
- schemas/environment.schema.json (draft 2020-12): name, account_id,
  region, state_backend, network, runner_role_arn, autonomy, confidence_threshold.
- core/environments/qa.json, prod.json, dr.json placeholder bindings
  (attested, thresholds 0.75/0.90/0.95, placeholder account_id with
  stderr warning at load).
- core/environment_check.py: load(env_name) helper + placeholder warning.

Sample contracts:
- contracts/static-assets.yaml + microservice.yaml use
  acdl-${env.environment}-${contract.module}-${env.account_id}-${env.region}
  naming pattern (region + account id + environment).

Tests: +35 (test_environment_schema.py, test_interpolation.py,
test_sample_contracts_interpolate.py). 406 passed; run_ci.sh green;
run_platform.sh --check-only green. Existing fixture-based tests
preserved (instance.json static fixtures unaffected).

core/environment_check.py

@@ -10,6 +10,7 @@ Usage:
     python3 core/environment_check.py <contract.yaml>
     python3 core/environment_check.py --env dev
 """
+import json
 import sys
 from pathlib import Path
 
@@ -32,6 +33,27 @@ def _contract_environment(contract_path):
     return contract.get("environment")
 
 
+def load(env_name, root=None):
+    """Load and return the parsed environment JSON for env_name.
+
+    Returns the env dict, or raises FileNotFoundError if no <env_name>.json
+    exists. Emits a stderr warning when account_id is the 000000000000
+    placeholder and env_name != 'dev' (prompts real binding).
+    """
+    env_file = _environments_dir(root) / f"{env_name}.json"
+    if not env_file.is_file():
+        raise FileNotFoundError(f"no environment file for '{env_name}' at {env_file}")
+    with open(env_file) as f:
+        env = json.load(f)
+    if env.get("account_id") == "000000000000" and env_name != "dev":
+        sys.stderr.write(
+            f"WARNING: environment '{env_name}' has the placeholder account_id "
+            f"000000000000 — replace it with the real {env_name} account id "
+            f"before deploying (onboarding scaffold).\n"
+        )
+    return env
+
+
 def _onboarding_message(env_name):
     return (
         "=== ACDL Environment Onboarding ===\n"