fix(proposals): catch id collision in check_approvable batch precheck

[jaytbarimbao-collab]

summary

approve() refuses to overwrite an artifact whose id already exists (guards against a crash-retry between put_<kind>() and move_proposal_to_decided() silently rewriting it), but check_approvable() — the read-only precheck vouch approve a b uses to validate a whole batch before mutating anything — never re-implemented that same guard.

what changed

so a claim/entity/relation proposal whose id collides with an existing artifact passes the precheck as approvable, then approve() raises on it anyway — breaking the documented all-or-nothing batch guarantee (a typo or already-decided id is supposed to abort the whole batch before anything mutates; instead the batch can partially apply).

factored the existing-artifact check into a shared, non-raising _existing_artifact_block_reason helper in src/vouch/proposals.py. approve()'s _ensure_no_existing_artifact now delegates to it, and check_approvable calls the same helper for the same set of kinds (all but page and delete — the same exception approve() already carves out).

test plan

new regression test test_check_approvable_catches_id_collision_with_existing_claim in tests/test_storage.py: approves a claim with id foo-bar, files a second pending claim proposal with the same id, asserts check_approvable now returns a non-None "already exists" reason (fails on the old code — returns None), and that approve() still raises on it.

.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.

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