fix(health): vouch fsck crashes with KeyError on any approved delete proposal

[philluiz2323]

What happened

vouch fsck crashes with an uncaught KeyError on any KB that has ever
had an approved delete proposal.

_check_decided_proposals in src/vouch/health.py builds a presence
dict keyed by ProposalKind, then indexes it by every approved
proposal's own kind:

# src/vouch/health.py:479-497 (before fix)
presence: dict[ProposalKind, set[str]] = {
    ProposalKind.CLAIM: set(claims),
    ProposalKind.PAGE: set(pages),
    ProposalKind.ENTITY: set(entities),
    ProposalKind.RELATION: relations,
}
for pr in store.list_proposals(ProposalStatus.APPROVED):
    artifact_id = pr.payload.get("id") if isinstance(pr.payload, dict) else None
    ...
    if artifact_id not in presence[pr.kind]:

presence has no entry for ProposalKind.DELETE. But propose_delete()
(src/vouch/proposals.py) files delete proposals with
kind=ProposalKind.DELETE, and approve() explicitly allows a DELETE
proposal to be approved without requiring the target artifact to
pre-exist (it's being removed, not created). Any approved delete
proposal reaching this loop hits presence[ProposalKind.DELETE] →
KeyError, aborting fsck() entirely.

What you expected

vouch fsck should handle DELETE proposals correctly: verify the
target artifact is actually absent (checked against target_kind, the
kind of what was deleted — not pr.kind, which is always DELETE), and
report a Finding if it's invalid or wasn't actually removed, rather than
crashing.

Reproduction

import tempfile, pathlib
from vouch.storage import KBStore
from vouch import proposals, health

d = pathlib.Path(tempfile.mkdtemp())
store = KBStore.init(d)
src = store.put_source(b"evidence", title="s")
pr = proposals.propose_claim(store, text="the sky is blue", evidence=[src.id], proposed_by="agent-a")
claim = proposals.approve(store, pr.id, approved_by="human-b")

del_pr = proposals.propose_delete(store, target_kind="claim", target_id=claim.id, proposed_by="agent-a")
proposals.approve(store, del_pr.id, approved_by="human-c")

health.fsck(store)  # raises

Output:

Traceback (most recent call last):
  File ".../health.py", line 369, in fsck
    _check_decided_proposals(store, claims, pages, entities, findings)
  File ".../health.py", line 497, in _check_decided_proposals
    if artifact_id not in presence[pr.kind]:
KeyError: <ProposalKind.DELETE: 'delete'>

vouch fsck's CLI entry (src/vouch/cli.py) calls health.fsck() with
no exception handling around it, so the CLI command itself crashes.

tests/test_health.py has zero coverage of delete proposals in
_check_decided_proposals, which is why this went unnoticed.

Environment

• vouch version: test branch @ current HEAD
• Python version: 3.11+
• OS: any
• Host: any — vouch fsck / kb.fsck (if exposed) is host-independent

.vouch/ state

Not required to reproduce — the repro above uses a fresh temp KB.

Anything else

This exact defect and a fix were previously submitted as #538
(fix(health): fsck survives approved delete proposals,
CodeRabbit-reviewed, author addressed review feedback) but the PR was
closed unmerged on 2026-07-29 purely for going stale against a
fast-moving test branch (a CHANGELOG.md conflict) — the maintainer's
closing comment explicitly said "this isn't a judgement on the change
itself... happy to look again." Re-verified independently against
current test HEAD: the crash is still live.

Suggested fix: handle ProposalKind.DELETE proposals in a first pass —
resolve target_kind from the payload, report
decided_delete_invalid_target_kind if it's missing/unrecognized, report
decided_delete_artifact_present if the target still exists on disk, and
otherwise record the id as legitimately deleted. A second pass then
checks ordinary create/edit proposals against presence, skipping any
artifact id that a delete proposal legitimately removed — otherwise the
original creating proposal (still recorded as APPROVED in decided/)
false-positives as decided_missing_artifact once its artifact is
correctly deleted.