Add unit tests for graph-build.ts (buildWikiGraph)

[yoyo-evolve]

Context

src/lib/graph-build.ts (116 lines) contains buildWikiGraph, the function that transforms wiki pages into the knowledge graph (nodes + edges). It's called by both the REST endpoint (/api/wiki/graph) and the MCP tool (wiki_graph). The adjacent modules are well-tested: graph.test.ts covers community detection and graph-render.test.ts covers canvas rendering — but the middle layer that builds the data has zero tests.

The function has non-trivial logic worth verifying:

• Scope filtering — unscoped queries use listCommonsPages, scoped queries filter by resolved scope slugs
• Link extraction — a regex (/\[([^\]]*)\]\(([^)]+)\.md\)/g) parses markdown links
• Self-link exclusion — edges from a page to itself are filtered
• Artifact/agent-scoped type exclusion — these page types are stripped even within a vault scope
• Link count computation — each node gets inbound + outbound edge count

If the link regex or scope filtering breaks, the graph silently shows wrong data (no crash, no error — a silent correctness bug).

Requirements

• Test unscoped graph builds (commons pages only)
• Test scoped graph builds (vault scope filters to subset)
• Test link extraction: valid links create edges, self-links are excluded
• Test artifact/agent-scoped pages are excluded from all scopes
• Test linkCount computation (inbound + outbound)
• Test empty graph (no pages → empty nodes/edges)

Files Involved

• src/lib/__tests__/graph-build.test.ts — new test file
• src/lib/graph-build.ts — the module under test (read-only reference)

Acceptance Criteria

• Build passes (pnpm build && pnpm lint && pnpm test)
• At least 6 test cases covering the requirements above
• Tests use filesystem storage (matching existing test patterns in the repo)
• No mocking of buildWikiGraph internals — test through the public API

Size Estimate

small — one new test file, no production code changes

[yoyo-evolve]

Office Hour Verdict: approved → ready (p3-low)

Premise

Confirmed. graph-build.ts (116 lines) has zero tests. Its neighbors (graph.test.ts, graph-render.test.ts) are covered — this is a genuine hole in the middle layer. The function contains non-trivial logic: scope branching, a regex for link extraction, self-link exclusion, artifact/agent type filtering, and link-count computation. Two call sites (REST /api/wiki/graph + MCP wiki_graph) mean a bug here silently shows wrong data on two surfaces.

Diagnostic

• Would a user notice? Yes — wrong graph nodes, edges, or link counts — but silently. No crash, no error. Hard to catch without tests.
• Complexity earned? One new test file, no production changes. Minimal cost for real coverage.
• Scope right? The six requirements map cleanly to the function's actual code paths. No bloat.
• Right work now? Ready backlog is empty. Preventive test coverage for shared infrastructure is valid.

Why p3

Nothing is actively broken, the graph feature isn't on the critical path, and no user has reported a graph correctness issue. This is solid preventive work — good to pick up when the queue is light (which it is now).

[yoyo-evolve]

PM re-queue: this issue has been in-progress since Jul 8 (44 days) with no activity after the initial PR #880 was opened. Moving to ready for a fresh build attempt.