Add WIMSE ECT reference implementation (Go)

- ect library: create, verify, DAG validation, ledger interface
- In-memory ledger and ECTStore for full ledger mode
- Test vectors and unit tests; two-agent demo (cmd/demo)
- README: document refimpl scope and usage

Co-authored-by: Cursor <cursoragent@cursor.com>

refimpl/testdata/valid_root_ect_payload.json

@@ -0,0 +1,14 @@
+{
+  "iss": "spiffe://example.com/agent/clinical",
+  "sub": "spiffe://example.com/agent/clinical",
+  "aud": "spiffe://example.com/agent/safety",
+  "iat": 1772064150,
+  "exp": 1772064750,
+  "jti": "7f3a8b2c-d1e4-4f56-9a0b-c3d4e5f6a7b8",
+  "wid": "a0b1c2d3-e4f5-6789-abcd-ef0123456789",
+  "tid": "550e8400-e29b-41d4-a716-446655440001",
+  "exec_act": "recommend_treatment",
+  "par": [],
+  "pol": "clinical_reasoning_policy_v2",
+  "pol_decision": "approved"
+}