feat(graph): add a labeled relation graph with a query surface

[drewstone]

Why

buildKnowledgeGraph is hardcoded to page links and keys edges by source->target, so two predicates between one pair collapse into one edge with merged reasons. KnowledgeRelation has an arbitrary predicate, but nothing builds a graph from it and nothing queries one. A consumer with labeled provenance (runs, claims, models; branched-from, supersedes, cites-evidence) had to hand-roll the builder and every query.

What

• src/relation-graph.ts: buildKnowledgeRelationGraph({ nodes?, relations }) builds a KnowledgeRelationGraph with one edge per (sourceId, targetId, predicate) in first-seen order. A repeated triple is accepted only when its weight and metadata are byte-identical in canonical JSON; any other repeat, an endpoint outside declared nodes, a duplicate node id, an empty id/kind/predicate, a non-finite weight, or non-JSON metadata is refused with a typed KnowledgeRelationGraphError (code). Without nodes, the graph carries edges only.
• neighbors(graph, id, { predicate?, direction }), walk(graph, id, { predicate?, direction, maxDepth? }) (breadth-first, visited set, cycle-safe; ancestors = out over branched-from, descendants = in), isReachable(graph, from, to, { predicate?, direction }). An unknown node or malformed query is refused, never an empty answer.
• KnowledgeRelationSchema (hoisted out of KnowledgeBaseCandidateSchema, one definition), KnowledgeRelationNodeSchema, KnowledgeRelationGraphSchema, each with an explicit metadata: z.record(z.string(), z.unknown()).optional() so a persisted graph round-trips.
• knowledgePageRelations(pages) emits the labeled page relations (wikilink, citation, shared-source, contradicts), one per triple with occurrence weights. buildKnowledgeGraph now collapses that list into its weighted edges (one source of truth); contradicts stays out of the collapsed projection and the implicit shared-source pass stays out of the relation graph.
• Exported from the entrypoint; README and docs/architecture.md lines; CHANGELOG 8.1.0; api-surface.json regenerated (20 added exports, additive → minor).

Proof (local, macOS, Node 24.11.1)

• pnpm run typecheck: pass (src + contracts).
• pnpm run lint: 227 files, no fixes.
• pnpm test: 672 passed, 16 skipped, 48 failed — all 48 in tests/kb-improvement/* (6 files), which throw exact knowledge candidate workflows require Linux directory descriptors on macOS; unrelated to this change. Focused: src/relation-graph.test.ts 11/11, src/graph.test.ts 5/5, src/citation-storage.test.ts 4/4, tests/core.test.ts + src/search.test.ts 24/24.
• Golden test: buildKnowledgeGraph(pages) over a fixture covering double-resolved links, origin-qualified and malformed citations, ambiguous duplicate ids, self links, shared sources, and contradicts is JSON.stringify-identical to the output recorded from the pre-refactor implementation (key order, edge order, weights, degrees).
• pnpm run build: 35 files. pnpm api:surface: 926 exports across 6 entry points. pnpm run check:version-bump: 20 export changes needing a minor bump, paid for by 8.0.10 -> 8.1.0.
• git merge-tree --write-tree origin/main HEAD: clean.

Closes #124

[tangletools]

✅ Auto-approved drewstone PR — 9b6870b3

This PR was opened by the trusted drewstone account.

This approval is provisional and was applied by the local stand-in because the pr-reviewer webhook host is unreachable (2026-08-21). CI on this head is fully green. The full PR reviewer audit re-runs via the resweep when the service returns and will publish findings if it detects issues.