docs(functions): split RETURNS into EXIT STATUS and stdout RETURNS

RETURNS previously conflated fish's $status exit code with genuine
stdout/printed output, e.g. rm listing "0/1" as if they were print
values rather than exit codes. Rename RETURNS to EXIT STATUS across
all 83 documented functions, and reintroduce RETURNS as a distinct
label reserved for the 15 functions that actually print to stdout.

Update build-manual.py's ENTRY_HEADS to render Exit Status before
Returns, manualtools.py's SECTIONS constant, and AGENTS.md's label
order and label-usage guidance to match. Add two verify-manual.py
regression tests: EXIT STATUS bodies must never contain stray
stdout/printed language, and Returns: must always render after
Exit Status: when both are present. Regenerate docs/fish-config.md.

functions/repo-open.fish

@@ -37,10 +37,13 @@
 #   -r, --root   Ignore the current sub-directory; link to the repo root
 #   -h, --help   Print usage and exit
 #
-# RETURNS
-#   0  URL opened (or printed)
+# EXIT STATUS
+#   0  URL opened, or resolved with -p/--print
 #   1  Not a git repo, no origin remote, or browser launch failed
 #
+# RETURNS
+#   With -p/--print, the resolved repository URL, printed to stdout
+#
 # EXAMPLE
 #   repo-open              # open current branch (+ subdir) in browser
 #   repo-open --print      # just print the URL