docs,feat(doc-gates): fix 15 dead anchor fragments; verify-md-links now validates fragments

A corpus sweep under the doc/prose standards found 15 links whose #fragment
named no anchor in its target — reworded headings, one relocated contract
(tool-fs → the group README's no-timeout rule), and zh sides citing English
slugs their Chinese headings never produce. Fixed all 15 (zh sides get the
conventional explicit <a id> + English fragment), fixed the one generator-owned
instance at its source (gen-doc-graphs), and extended verify-md-links to
resolve fragments onto Markdown targets — same-file anchors included — against
heading slugs and explicit <a id>, so the class is gated instead of manually
grepped. Remaining probes (narrated history, duplication shingles, comment
transcripts, budgets) came back clean; sibling-adapter README symmetry and
implemented-note contrasts are deliberate keeps.

docs/AGENTS.md

@@ -72,6 +72,6 @@ Hunt these in any doc; the [dsh-doc-standards](../.agents/skills/dsh-doc-standar
 
 ## Cross-reference with machine-checkable links, never free prose
 
-Link repository references with relative Markdown paths, never bare filenames or Agent Note numbers. `verify-md-links` catches missing targets; the [cross-link Agent Note](../.agents/notes/implemented/process/2026-06-18-markdown-cross-link-lint.md) owns the rationale.
+Link repository references with relative Markdown paths, never bare filenames or Agent Note numbers. `verify-md-links` rejects missing targets and dead `#fragment` anchors ([rationale](../.agents/notes/implemented/process/2026-06-18-markdown-cross-link-lint.md)).
 
 The gate checks file existence, not `#anchor` validity — verify anchors yourself when linking to one.