fix(lifecycle): reject claim self-contradiction

[davion-knight]

summary

lifecycle.contradict() had no guard against claim_a == claim_b, unlike its sibling supersede(), which explicitly rejects old_claim_id == new_claim_id. calling contradict() with the same id on both sides wrote a self-loop contradicts reference onto the claim and flipped its status to contested with no actual counterparty — durable, nonsensical on-disk state with no way to recover short of manual yaml editing. all three surfaces (cli, mcp, jsonl) reach the unguarded function with zero pre-checks.

what changed

added the same self-reference guard supersede() already has, at the top of contradict() in src/vouch/lifecycle.py.

test plan

new regression test test_contradict_rejects_self_reference in tests/test_storage.py — asserts LifecycleError is raised and the claim is left untouched on disk (fails on the old code, which raised nothing and corrupted the claim).

.venv/bin/python -m ruff check src tests
.venv/bin/python -m mypy src
.venv/bin/python -m pytest tests/ -q --ignore=tests/embeddings

all three clean on the rebased branch (rebased onto test after the recent CI incident was resolved).

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