fix(kb-lint): a markdown link may not resolve into the quarantine

The link-integrity pass ended in `is_file()`, and `.AGENTTRASH/` is a real directory on
this machine: a page pointing at `[x](.AGENTTRASH/gone.md)` passed the gate here and was a
dead link everywhere else, because the quarantine is gitignored. This is incident #57 — the
wikilink resolver resolving `[[log]]` onto a quarantined copy — reproduced in markdown
clothing, which is to say: fixing one resolver while leaving the identical assumption in the
other one is not a fix.

A link that lands inside the bundle must land on a page that belongs to the bundle. A link
that leaves the bundle is still honoured (`tools/live-sync.md` -> `../../AGENTS.md` points at
the operating contract, which is not bundle content and is real on disk). Red test added
first, both link spellings covered; nothing previously green changed.

Refs #64

tests/kb-lint.bats

@@ -134,6 +134,21 @@ EOF
   [[ "$output" == *"broken wikilink -> [[log]]"* ]]
 }
 
+@test "a markdown link into the quarantine is broken" {
+  # #57's failure mode wearing markdown instead of brackets: the relative-link check was a
+  # bare is_file(), so a link that lands on a page waiting in .AGENTTRASH/ passes this gate
+  # and points at nothing on every machine that pulls — the quarantine is gitignored. Both
+  # spellings count: relative, and bundle-absolute with a leading slash.
+  mkdir -p "$KB/.AGENTTRASH"
+  printf 'no frontmatter, quarantined\n' > "$KB/.AGENTTRASH/gone.md"
+  printf -- '---\ntype: reference\n---\nRelative: [gone](.AGENTTRASH/gone.md)\n' > "$KB/rel.md"
+  printf -- '---\ntype: reference\n---\nAbsolute: [gone](/.AGENTTRASH/gone.md)\n' > "$KB/abs.md"
+  run bash "$LINT" "$KB"
+  [ "$status" -ne 0 ]
+  [[ "$output" == *"rel.md: broken relative link -> .AGENTTRASH/gone.md"* ]]
+  [[ "$output" == *"abs.md: broken relative link -> /.AGENTTRASH/gone.md"* ]]
+}
+
 @test "a wikilink to a live page still resolves, in any directory" {
   # The narrowing above must cost nothing: filename-anywhere semantics is the point.
   printf -- '---\ntype: reference\n---\nSee [[good]].\n' > "$KB/sub/other.md"