fix(check-emphasis): recognise code fences opened inside a blockquote

FENCE_RE was `^\s*(`{3,}|~{3,})`, which matches leading whitespace but not
a blockquote marker. A fence written as `> ```python` was therefore never
recognised, so the quoted code was scanned as prose and CJK-adjacent `**`
inside it was reported as non-rendering emphasis. Since the emphasis check
became a blocking CI gate, that false positive could red-line a legitimate
push over a code sample in a callout.

This file documents itself as mirroring check 6 of the workspace-level
format_checker.py, whose FENCE_RE already handles up to three spaces of
indent plus any number of `>` prefixes. Adopting that pattern restores the
stated invariant.

Demonstrated before/after on a file holding the same line both inside and
outside a blockquoted fence: previously only the quoted copy was flagged
(exit 1); now neither is (exit 0), while a genuine prose defect is still
reported (exit 1), so the gate is not weakened. All 14 repos re-checked:
check_emphasis and check_project_rules pass on all 1,615 markdown files,
and the 14 copies remain byte-identical. A mutation-tested regression guard
now lives in the workspace repo at tests/test_blockquote_fence_emphasis.py.

check_emphasis.py

@@ -39,7 +39,7 @@ SKIP_DIRS = {
     "output",
 }
 
-FENCE_RE = re.compile(r"^\s*(`{3,}|~{3,})(.*)$")
+FENCE_RE = re.compile(r"^[ \t]{0,3}(?:>[ \t]?)*(`{3,}|~{3,})(.*)$")
 PUNCT_CATEGORIES = {"Pc", "Pd", "Pe", "Pf", "Pi", "Po", "Ps"}
 ASCII_PUNCT = set("!\"#$%&'()*+,-./:;<=>?@[\\]^_`{|}~")
 CJK_RE = re.compile(r"[㐀-鿿豈-﫿]")