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.

Makefile

@@ -699,11 +699,11 @@ endif
 # GitHub Actions linters are run in a separate CI job (lint-actions) that only
 # triggers when workflow files change, so we skip them here when CI=true.
 LINT_ACTIONS_TARGETS := $(if $(CI),,lint/actions/actionlint)
-lint: lint/shellcheck lint/go lint/ts lint/examples lint/helm lint/site-icons lint/markdown lint/check-scopes lint/migrations lint/bootstrap $(LINT_ACTIONS_TARGETS)
+lint: lint/shellcheck lint/go lint/ts lint/examples lint/helm lint/site-icons lint/markdown lint/check-scopes lint/migrations lint/bootstrap lint/emdash $(LINT_ACTIONS_TARGETS)
 .PHONY: lint
 
 # Subset of lint that does not require Go or Node toolchains.
-lint-light: lint/shellcheck lint/markdown lint/helm lint/bootstrap lint/migrations lint/actions/actionlint lint/typos
+lint-light: lint/shellcheck lint/markdown lint/helm lint/bootstrap lint/migrations lint/actions/actionlint lint/typos lint/emdash
 .PHONY: lint-light
 
 lint/site-icons:
@@ -738,6 +738,10 @@ lint/bootstrap:
 	bash scripts/check_bootstrap_quotes.sh
 .PHONY: lint/bootstrap
 
+lint/emdash:
+	bash scripts/check_emdash.sh
+.PHONY: lint/emdash
+
 
 lint/helm:
 	cd helm/