fix(graph): stop leaking edges to excluded neighbors

[joaovictor91123]

What changed

Reordered the loop in src/vouch/graph.py::find_neighbors so an edge is
only recorded once its other endpoint has been accepted into visited —
either just now, having passed the _node_kind/_neighbor_ok gate, or
already accepted in an earlier iteration.

Why

find_neighbors() appended an edge to the response before checking
whether its other endpoint passed the same retrievability/existence
checks that gate node inclusion (_neighbor_ok / _node_kind).
Superseded, archived, and redacted claims — and missing nodes — were
correctly excluded from nodes, but the edge pointing at them still went
out, so a response could contain an edge whose target referenced a
claim id the response itself said didn't exist. kb.neighbors shares
this code path across all three surfaces (MCP, JSONL, CLI), so the leak
was identical everywhere.

Confirmed with a repro: a live claim related to a superseded claim
returns nodes: [] (correctly excluding the superseded claim) but
edges: [{'source': 'live-claim', 'target': 'dead-claim', ...}] — a
dangling edge to a node the response itself says doesn't exist. The
existing lifecycle.supersede() path hits this identically: the
SUPERSEDES relation it creates between old and new claim leaked the
same way.

Fixes #716

What might break

Nothing for users with an existing .vouch/ directory — no on-disk
shape, kb.* method, or object model change. Behaviorally: kb.neighbors
responses no longer contain edges pointing at nodes excluded from the
same response — strictly the intended, documented contract (a caller
should never see an edge referencing an id the response says doesn't
exist or isn't retrievable).

VEP

Not applicable — no object model, kb.* method, on-disk layout, bundle
format, or audit-log shape change. A response-consistency fix inside the
existing kb.neighbors read path.

Tests

• Local make check-equivalent: ruff clean (src + tests); mypy
  clean on graph.py; all tests/test_graph.py cases pass (9
  pre-existing + 1 extended); broader graph/neighbor/context
  selection across the suite passes
• New / changed behaviour has a test — extended
  test_find_neighbors_excludes_superseded_claims to also assert
  result["edges"] == []; confirmed it fails on the pre-fix code
  (returns the dangling SUPERSEDES edge) and passes with the fix
• CHANGELOG.md updated under ## [Unreleased]