style: align Vale heading comments with the sentence-case standard

Fallout from rebasing onto master: Pulumi.HeadingSentenceCase and its
.vale.ini entry both described Title Case H1 as the convention, which now
contradicts AGENTS.md and STYLE-GUIDE.md. Restate the comments as what the
scoping actually is — sentence case is the standard at every level; the rule
stays scoped to H2+ so it doesn't flag the backlog of Title Case front-matter
titles. No rule behavior changes.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015KpCy5B47c2oNqEeX1Znwx

Author: claude

.vale.ini

@@ -20,7 +20,7 @@ TokenIgnores = ({{[%<].*?[%>]}}), \
 
 # Disable Google rules that conflict with Pulumi house style (brand.pulumi.com
 # is the source of truth; these mirror its writing-style/voice sections offline).
-Google.Headings = NO        # Replaced by Pulumi.HeadingSentenceCase, which scopes to H2+ only (Pulumi uses Title Case for the H1/page title). Nothing else enforced H2 case — markdownlint has no sentence-case rule.
+Google.Headings = NO        # Replaced by Pulumi.HeadingSentenceCase. The brand standard is sentence case at every level, but the rule is scoped to H2+ so it doesn't flag the backlog of existing Title Case H1s (front-matter `title`) during migration. Nothing else enforced H2 case — markdownlint has no sentence-case rule.
 Google.Quotes = NO          # Brand puts commas/periods OUTSIDE closing quotes (logical style); Google's rule forces them inside.
 Google.WordList = NO        # Google product-name overrides don't match Pulumi terminology.
 Google.We = NO              # Blanket first-person flagger; replaced by Pulumi.NarrativeWe, which targets only narrative-walkthrough constructions (let's / we'll / now we ...), not every 'we'.

styles/Pulumi/HeadingSentenceCase.yml

@@ -2,10 +2,11 @@ extends: capitalization
 message: "Heading '%s' should use sentence case (capitalize only the first word and proper nouns)."
 link: STYLE-GUIDE.md
 level: warning
-# H2+ only. Pulumi convention is Title Case for the H1/page title and sentence
-# case for every subheading; Vale's per-level heading scopes let us enforce the
-# latter without touching H1. (Google.Headings is disabled in .vale.ini because
-# it has no level scope and would false-flag every Title-Case H1.)
+# H2+ only. The brand standard is sentence case at every level, page title
+# included, but enforcing it on H1 would flag the large backlog of existing
+# Title Case front-matter titles; Vale's per-level heading scopes let us
+# enforce the subheadings now and migrate H1s separately. (Google.Headings is
+# disabled in .vale.ini because it has no level scope.)
 scope:
   - heading.h2
   - heading.h3