The contributor entry docs link into trees the split moved, and the link guard is scoped past them

Context

Surfaced while handing #17855 to @vansh2408, an external contributor — the first community pickup in a while, and the repo's only open good first issue. Before handing over I read what a newcomer actually reads. CONTRIBUTING.md sends them to two files that a fresh clone does not have.

This is the third member of a family I already own two thirds of:

#17809's fix was a guard — buildScripts/util/check-relative-links.mjs, wired by .github/workflows/check-relative-links.yml. I wrote it, and I scoped its trigger to learn/**. That scope is why this went unseen: the guard that exists to notice dead relative links cannot see the single most-read contributor document in the repository.

The Problem

Measured over the 16 public entry docs (CONTRIBUTING.md, README.md, AGENTS.md, AGENTS_STARTUP.md, ROADMAP.md, BACKERS.md, .github/*.md), resolving every link relative to its containing document the way GitHub renders it, against git ls-files rather than a working tree:

repo-relative links checked : 31
dangling in a fresh clone   :  2   ← both in CONTRIBUTING.md
GitHub tab links excluded   :  3   (../../issues — GitHub rewrites these; not filesystem paths)

Controls: a doc-relative sibling (.github/WORKING_WITH_AGENTS.md → AI_QUICK_START.md) resolves true; a seeded fake resolves false.

The two, with their causes and their live replacements:

e2505e4681 names its own replacement in the commit subject: "Removing the old CODEBASE_OVERVIEW.md (replaced by learn/guides/fundamentals/CodebaseOverview.md)". The answer has been sitting in the history the whole time; nothing was watching the referrer.

The same section also cites `ai/mcp/server/neural-link/` as prose rather than a link. git ls-files ai/ returns 0 — that tree left for the Brain in c623b2f63c and is present locally only as an untracked artifact, so a fresh clone cannot follow it either. A link checker cannot see this one; it is not a link.

The Architectural Reality

Both deletions were correct. CODEBASE_OVERVIEW.md was consolidated into the portal-served guides; learn/agentos/** and ai/** are the Brain's hemisphere per ADR 0018. Restoring either file to the Engine would be the wrong fix. What is missing is that nothing updates a referrer when a custody decision moves its target — the trigger-starvation shape #17783 names for CI contexts, here in docs.

The Fix

1. Repoint ./.github/CODEBASE_OVERVIEW.md → learn/guides/fundamentals/CodebaseOverview.md, the replacement its own deletion commit names.
2. Repoint learn/agentos/StrategicWorkflows.md and the ai/mcp/server/neural-link/ prose reference to neomjs/neo-agent-brain, explicitly marked cross-repo. The corpus already has this convention — cross-repo paths carry an explicit repo prefix — it is simply unevenly applied.
3. Widen the existing guard rather than add one. check-relative-links.mjs already resolves and reports; it needs the root entry docs in its SCAN_ROOTS and its workflow paths: trigger, plus two rules the wider corpus requires.

Measured, by running the unmodified guard over the 16 entry docs:

NEO_CODE_BLOCK_1

• GitHub tab links. ../../issues resolves out of the tree and is reported. GitHub rewrites it to the repo's issues tab; it is not a filesystem path. Needs a rule, and the exclusion must be counted in the output rather than silently dropped.
• Portal classification must be scoped to learn/**. classifyTarget calls any slash-less, suffix-less target a portal ref and judges it against learn/tree.json. That is correct for a guide and wrong everywhere else: a <a href="LICENSE"> in a README would be checked against the portal manifest and reported dead. Zero occur in the corpus today, which is exactly why widening the scan set without this rule would plant a trap rather than trip one.

Contract Ledger

Decision Record impact

aligned-with ADR 0018 — the repoints mark the hemisphere boundary rather than crossing it.

Acceptance Criteria

• Every repo-relative link in the public entry docs resolves against git ls-files — the fresh-clone view, not a working tree
• The ai/mcp/server/neural-link/ reference in CONTRIBUTING.md is marked cross-repo or removed; no bare Engine-relative form survives
• A slash-less, suffix-less target outside learn/** (e.g. LICENSE in a README) is resolved as a path, not judged against learn/tree.json; a spec pins it and must fail without the scoping
• GitHub tab links (../../issues) are excluded by rule and the exclusion is counted in the guard's output, not silently dropped
• The guard reds on a seeded dangling link in CONTRIBUTING.md — red control required; a checker that only ever passes is not evidence
• The workflow triggers on changes to the entry docs and to the guard's own files, so a checker that stops working is caught by the trigger that would have caught the links
• learn/** coverage from #17809 is unchanged — the existing behaviour keeps its tests

Out of Scope

• The ai/ prose class in AGENTS_STARTUP.md (6 references) and .github/AI_QUICK_START.md (5). Measured, and left alone deliberately: those documents describe a genuinely Brain-side workflow (worktree bootstrap, MCP server config, Memory Core). Whether the Engine should carry them at all, repoint them cross-repo, or drop them is a disposition decision, not a mechanical repoint — and it overlaps neomjs/neo-agent-skills#24's territory. Repointing them silently here would be the wrong call made quietly.
• #17877's portal renderer work. Different surface, different fix; these docs are not portal-served.
• #17783's missing required status contexts and guards-without-callers. Adjacent shape — this is a guard with a caller and too narrow a trigger — but neither closes the other.
• Any restoration of deleted content into the Engine.

Avoided Traps

• Measuring the working tree instead of the repo. ai/mcp/server/neural-link/ exists on my disk and is tracked by nothing. A filesystem probe reports it healthy; git ls-files reports the truth a newcomer gets. This is the same working-tree-vs-repo error that has burned this class before.
• Reporting the first measurement. My initial sweep said 10 dangling. Eight were my throwaway script resolving document-relative links against the repo root and treating GitHub's ../../issues convention as a filesystem path. The real count is 2.
• Correcting my own ticket on the durable surface. The first version of this body said the checker itself lacked document-relative resolution and "must gain" it. That was false — collectDeadLinks has always resolved against path.posix.dirname(file); the bug was in the throwaway script I measured with, and I attributed my instrument's defect to the guard I was proposing to change. Caught by running the real guard instead of reasoning about it. The Fix and Contract Ledger above are the corrected text; this note stays because editing a body does not erase what it claimed.
• Filing this as new. It is the third increment of #17809/#17877, and the guard whose scope let it through is my own. Naming that is the point: a defect class recurs within arm's reach of its own fix.
• Restoring the deleted files. Both deletions were correct; only the referrers are stale.

Live latest-open sweep: checked the latest 20 open Engine issues at 2026-08-31T20:54:06Z, plus a keyword sweep (link OR dangling OR CONTRIBUTING OR onboarding) across open issues; nearest neighbours are #17877 (mine, portal renderer) and #17783 (@neo-gpt, enforcement residuals), neither equivalent. A2A in-flight sweep over the latest 10 messages: no overlapping [lane-claim].

Origin Session ID: bd974a9a-cef5-4d7b-99e5-9329b3f6f053

Retrieval Hint: query_raw_memories("CONTRIBUTING dangling links root entry docs link guard scope newcomer")

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