Extend the instructions gate to contributor docs

[FelixTJDietrich]

Contributor docs assert things about the repository that stop being true, and nothing notices.

Most of this issue has been overtaken. #1518 shipped scripts/check-agent-instructions.ts, which already covers the agent-file half — including a dangling check for an @ reference resolving to nothing. It deliberately scopes itself to "directories holding markdown addressed to an agent rather than to a reader", so docs/contributor/** is untouched by it.

Of the five instances that motivated this issue, zero remain: four were fixed by #1472, and the .agent/workflows/ reference was fixed by #1518. The two surviving scripts/codex-setup.sh references in dockerless-postgres.mdx belong to #1503, which deletes both the script and the guide.

So this is now purely preventive, and it should be an extension rather than a new script.

Scope

Extend scripts/check-agent-instructions.ts to a second file set — docs/contributor/**, *.md at the repo root — applying only the checks that make sense for prose addressed to a reader:

• a backtick-quoted string that looks like a repo-relative path must resolve
• a backtick-quoted string that looks like an npm package name must be declared in some package.json

Reuse the existing markdown scanner. #1518's PR notes that its scanner had three parsing bugs its tests did not catch, because the tests covered the rules and not the parsing underneath — so add cases at the parsing layer, not only at the rule layer.

Do not extend into prose linting. Paths and package names only: checkable without judgement, and the only thing that actually broke.

Sequencing

After #1503, which removes the two remaining live instances. Starting before that means writing an allowlist entry you delete a week later.

Done when

• pnpm run check fails when a contributor doc names a file or package that does not exist
• the check lives in the existing gate, not a second script
• the allowlist is short and every entry says why

[FelixTJDietrich]

#1495 removes the duplicate agent-instruction copies; this check is what stops them drifting back.

[FelixTJDietrich]

Correction to the original body: four of the five instances were already fixed by #1472 before this issue was filed. Only the .agent/workflows/ reference is still live. The issue now says so — the argument is recurrence prevention, not a backlog.

[FelixTJDietrich]

Scope note after #1495 was rewritten: #1495 removes the duplicated content (one source, tool-native pointers), so copy-drift stops being possible. This issue covers a different failure — docs naming a path or package that does not exist — which pointers do not prevent. Still worth doing, but the overlap is smaller than it looked: build it after #1495 settles the file set.

[FelixTJDietrich]

Re-checked on main after #1479. Of the five original instances, one remains: docs/contributor/ai-agent-workflow.mdx still points at .agent/workflows/. The ESLint + Prettier claim in local-development.mdx is fixed.

Two new ones exist in docs/contributor/dockerless-postgres.mdx, which names scripts/codex-setup.sh twice — #1503 removes both the script and the guide, so leave them to that issue.