chore: add no-emdash/endash rule to agent instructions and CI lint (#24375)

Add a lint check that prevents introduction of Unicode emdash (U+2014)
and endash (U+2013) characters. These are almost exclusively introduced
by AI agents and conflict with the project writing style.

The lint script (scripts/check_emdash.sh) checks only added lines in
the current diff by default, so existing violations do not block CI.
Pass --all to scan the entire repo for auditing.

Agent instructions in AGENTS.md, site/AGENTS.md, and the docs style
guide now explicitly ban emdash, endash, and " -- " as punctuation,
with guidance to use commas, semicolons, or periods instead.

scripts/check_emdash.sh

@@ -0,0 +1,113 @@
+#!/usr/bin/env bash
+set -euo pipefail
+# shellcheck source=scripts/lib.sh
+source "$(dirname "${BASH_SOURCE[0]}")/lib.sh"
+cdroot
+
+echo "--- check for emdash/endash characters"
+
+mode="changed"
+for arg in "$@"; do
+	if [[ "$arg" == "--all" ]]; then
+		mode="all"
+	fi
+done
+
+# Build the pattern from raw bytes so the script itself does not
+# contain literal emdash/endash characters (which would trigger
+# the check when the script is in the diff).
+emdash=$'\xE2\x80\x94'
+endash=$'\xE2\x80\x93'
+pattern="${emdash}|${endash}"
+
+scan_all_files() {
+	local output
+	output=$(git ls-files -z | xargs -0 grep -IEn "$pattern" 2>/dev/null || true)
+	if [[ -n "$output" ]]; then
+		echo "$output"
+		found=1
+	else
+		found=0
+	fi
+}
+
+if [[ "$mode" == "all" ]]; then
+	scan_all_files
+else
+	base=""
+	if [[ -n "${GITHUB_BASE_REF:-}" ]]; then
+		base="origin/${GITHUB_BASE_REF}"
+	elif git rev-parse --verify origin/main >/dev/null 2>&1; then
+		base=$(git merge-base HEAD origin/main 2>/dev/null || echo "origin/main")
+	fi
+
+	if [[ -z "$base" ]]; then
+		echo "WARNING: no base ref found, scanning all tracked files."
+		scan_all_files
+	else
+		# Ensure the base ref is fetchable. CI shallow clones
+		# (fetch-depth: 1) may not have the base branch available.
+		if ! git rev-parse --verify "$base" >/dev/null 2>&1; then
+			ref="${base#origin/}"
+			echo "Base ref $base not found locally, fetching $ref..."
+			git fetch origin "$ref" --depth=1 2>/dev/null || true
+			if ! git rev-parse --verify "$base" >/dev/null 2>&1; then
+				echo "ERROR: could not fetch base ref $base."
+				exit 1
+			fi
+		fi
+
+		found=0
+		if ! diff_output=$(git diff "$base" -U0 -- . 2>&1); then
+			echo "ERROR: git diff against $base failed:"
+			echo "$diff_output"
+			exit 1
+		fi
+
+		if [[ -z "$diff_output" ]]; then
+			echo "OK: no changes to check."
+			exit 0
+		fi
+
+		# Parse the diff to check only added lines for emdash/endash.
+		current_file=""
+		current_line=0
+		while IFS= read -r diff_line; do
+			if [[ "$diff_line" =~ ^\+\+\+\ b/(.*) ]]; then
+				current_file="${BASH_REMATCH[1]}"
+			fi
+			# Anchored to hunk header structure to avoid matching
+			# digits from trailing function context.
+			if [[ "$diff_line" =~ ^@@\ -[0-9,]+\ \+([0-9]+) ]]; then
+				current_line=${BASH_REMATCH[1]}
+				continue
+			fi
+			if [[ "$diff_line" =~ ^\+ ]] && [[ ! "$diff_line" =~ ^\+\+\+\ [ab/] ]]; then
+				if echo "$diff_line" | grep -Eq "$pattern"; then
+					echo "${current_file}:${current_line}:${diff_line:1}"
+					found=1
+				fi
+				((current_line++)) || true
+			fi
+		done <<<"$diff_output"
+	fi
+fi
+
+if [[ "$found" -ne 0 ]]; then
+	echo ""
+	echo "ERROR: Found emdash (U+2014) or endash (U+2013) characters."
+	echo ""
+	echo "  Do not use emdash or endash in code, comments, string literals,"
+	echo "  or documentation. Use commas, semicolons, or periods instead."
+	echo "  Restructure the sentence if needed. Do not replace them with"
+	echo "  ' -- ' either."
+	echo ""
+	echo "  Example:"
+	echo "    Bad:  This is slow [emdash] we should cache it."
+	echo "    Good: This is slow. We should cache it."
+	echo "    Good: This is slow, so we should cache it."
+	echo ""
+	exit 1
+fi
+
+echo "OK: no emdash or endash characters found."