docs: add per-tool API documentation and fix parity gaps

- Create 53 per-tool VitePress documentation pages with accurate
  parameters from Zod schemas, example requests, and response formats
- Add root llms.txt for LLM-friendly repo browsing
- Fix OpenAPI spec: add auth and 422 error schemas to
  edit-metadata/inspect and strip-metadata/inspect sub-routes
- Fix tool count inconsistency (52 -> 53) across landing site,
  e2e tests, and local docs
- Rename color-adjustments.ts to adjust-colors.ts to match tool ID
- Update VitePress sidebar with all 8 tool categories and top nav

tests/e2e-docs/homepage.spec.ts

@@ -29,7 +29,7 @@ test.describe("Docs Homepage", () => {
 
   test("features section renders all 6 feature cards", async ({ page }) => {
     const features = [
-      "52 Image Tools",
+      "53 Image Tools",
       "Local AI",
       "Pipelines",
       "REST API",