feat: migrate refimpls from draft-00 to draft-01 claim names

- Rename `par` to `pred` (predecessor) in types, serialization, tests
- Remove `pol`, `pol_decision` from core payload; move to `ect_ext`
- Remove `sub` from payload (not part of ECT spec)
- Update `typ` from `wimse-exec+jwt` to `exec+jwt` (accept both)
- Rename MaxParLength to MaxPredLength everywhere
- Update testdata, demos, READMEs with migration table
- All Go tests pass, all 56 Python tests pass (90% coverage)

refimpl/python/ect/ledger.py

@@ -23,7 +23,7 @@ class LedgerEntry:
     task_id: str
     agent_id: str
     action: str
-    parents: list[str]
+    predecessors: list[str]
     ect_jws: str
     signature_verified: bool
     verification_timestamp: float
@@ -70,7 +70,7 @@ class MemoryLedger(Ledger):
                 task_id=payload.jti,
                 agent_id=payload.iss,
                 action=payload.exec_act,
-                parents=list(payload.par) if payload.par else [],
+                predecessors=list(payload.pred) if payload.pred else [],
                 ect_jws=ect_jws,
                 signature_verified=True,
                 verification_timestamp=now,