feat(engine): LINK-FILE-URL — a link into someone's home directory

Second of the approved §5 checks. A `file:///Users/ktg/...` link is dead
for every reader but its author, and it publishes that author's directory
layout on a surface meant for strangers.

The SCHEME is not the rule, and measuring first is why. Across the corpus
there are 40 such links, and they split 18/22: a documented convention
example (`[Brief](file:///Users/ktg/.../brief.html)` — the same two lines
copy-pasted into nine CLAUDE.md files) versus real machine paths. Firing
on `file:` would have been wrong 45% of the time on its first run, which
is the ratio that gets a gate switched off. A further 22 links use bare
placeholders (`file:///abs/path.html`) and are not leaks either.

The discriminator is not tuned to this corpus: `...` is not a path
segment, so a target containing `/.../` cannot resolve on ANY machine and
is by construction an illustration.

Level follows the established reader rule — root is the shop window
(ERROR), below it live session plans and agent working files (WARN).

Measured on the registered corpus: exactly one finding, guard's
CLAUDE.md:56, a real absolute path to a file that is IN the repo and
should have been linked as `docs/BRIEF.md`. Every one of the 18
illustrations stayed silent.

Honest limit: three repos holding 21 of the 22 real leaks
(from-ai-to-chitta, wiki-advise, claude-code-llm-wiki) are NOT in the
register, so no check runs against them at all. That is register
freshness (`--refresh`), not this check.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Lb7XmJGLnFSX9U7tgS7fKk

scripts/repo-standard-check.test.mjs

@@ -758,6 +758,51 @@ test('org-profile requires no headings at all', () => {
   assert.equal(f.filter((x) => x.level === 'ERROR').length, 0);
 });
 
+// ------------------------------------------------------------- file: URL links
+
+// A `file:///Users/ktg/...` link is dead for every reader but its author, and
+// it publishes the author's directory layout. The scheme alone is NOT the rule:
+// measured across the corpus, 40 such links split 18/22 between a documented
+// convention example (`[Brief](file:///Users/ktg/.../brief.html)`, the same two
+// lines copy-pasted into nine CLAUDE.md files) and real machine paths (22, in
+// three repos). Firing on the scheme would have been wrong 45% of the time on
+// the first run — the ratio that gets a gate switched off.
+//
+// The discriminator is not tuned to this corpus: `...` is not a real path
+// segment, so a path containing `/.../` cannot resolve on ANY machine and is by
+// construction an illustration.
+test('a file: URL naming a real home directory is a finding', () => {
+  const files = { 'README.md': '[my notes](file:///Users/ktg/repos/x/notes.md)' };
+  const f = checkInternalLinks({ files, present: ['README.md'] });
+  const hit = f.find((x) => x.code === 'LINK-FILE-URL');
+  assert.equal(hit.level, 'ERROR');
+  assert.equal(hit.bucket, 'broken');
+  assert.match(hit.msg, /README\.md:1/);
+});
+
+test('an elided file: path is an illustration, not a leak — it resolves nowhere by construction', () => {
+  const files = { 'README.md': '[Brief](file:///Users/ktg/.../brief.html)' };
+  assert.equal(checkInternalLinks({ files, present: ['README.md'] }).some((x) => x.code === 'LINK-FILE-URL'), false);
+});
+
+test('a placeholder file: path is not a leak either', () => {
+  const files = { 'README.md': '[Open in browser](file:///abs/path.html)' };
+  assert.equal(checkInternalLinks({ files, present: ['README.md'] }).some((x) => x.code === 'LINK-FILE-URL'), false);
+});
+
+// Same reader rule as every other link: the root is the shop window, below it
+// live session plans and agent working files.
+test('a file: URL below the root is a WARN, not an ERROR', () => {
+  const files = { 'docs/plan.md': '[notes](file:///home/ktg/notes.md)' };
+  const f = checkInternalLinks({ files, present: ['docs/plan.md'] });
+  assert.equal(f.find((x) => x.code === 'LINK-FILE-URL').level, 'WARN');
+});
+
+test('other schemes stay somebody else\'s to resolve', () => {
+  const files = { 'README.md': '[spec](https://example.com/x) [editor](vscode://file/x)' };
+  assert.equal(checkInternalLinks({ files, present: ['README.md'] }).some((x) => x.code === 'LINK-FILE-URL'), false);
+});
+
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 // A lightweight tag is a branch-like ref: it can be moved to a different commit