Fix content issues flagged by CEO and QA (PR #42 review)

Critical fixes:
- stores.md: Correct supported retailers to Meijer, Kroger, Target.
  Remove Safeway (never scoped). Replace named Coming Soon list with
  generic demand-based evaluation language.
- privacy.md: Replace all OAuth/API claims with accurate language
  describing read-only headless browser access to loyalty portals.
- about.md: Remove "price gouging on our roadmap" claim.
  Clarify USDA FoodData Central is reference data only, not a source
  of price data.
- blog/price-gouging-vs-shrinkflation.md: Remove roadmap claim.
  Remove implication that price gouging detection is coming.
- methodology.md: Fix cereal example math — 16.2% → 16.1%.
  Use raw values per the stated formula. Clarify USDA FoodData
  Central role for package sizing baselines only.
- how-it-works.md: Correct retailers. Remove "(yet)" from receipt
  claim. Clarify USDA FoodData Central is reference data.

Important fixes:
- press-kit.md: Correct supported stores. Remove USDA FoodData Central
  from dollar-cost attribution — reattribute to CartSnitch analysis of
  manufacturer packaging data.
- app-store-listing.md: Remove "thousands of products" claims
  (pre-launch beta, quantity unverified).
- social/launch-day-posts.md: Remove "thousands of products" claim.
  Correct retailer list.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

content/marketing/privacy.md

@@ -40,7 +40,7 @@ We **do not store**:
 ## What We Never Do
 
 - **We never sell your data.** Your data is never a product. We don't license it, share it with third parties, or use it for advertising.
-- **We never see your login credentials.** Our store integrations use secure OAuth connections — we never have access to your store password.
+- **We never see your login credentials.** We use a read-only connection to your store loyalty account — we never have access to your store password.
 - **We never post to your social accounts or profile.**
 - **We never use your purchase data for anything other than the CartSnitch service.**
 
@@ -70,7 +70,7 @@ That's it.
 ## Security
 
 - All data is encrypted in transit and at rest
-- We use industry-standard OAuth for store connections — never your store password
+- We use read-only connections to store loyalty accounts — we never see your store password
 - Our team follows strict access controls — only the engineers who need your data to build the product can access it
 
 ---