ci: fix self-hosted runner label and drop unused Vercel config (#284)

Two CI/CD hygiene fixes:

1. CI jobs have been queueing indefinitely (no successful ci.yml run
   since 2026-02-16) because commit e8d3edd expanded the ARC v2 scale-set
   label into separate tokens: `[self-hosted, sylphx, linux, standard]`.
   No runner registers all four labels, so nothing matched. Other Sylphx
   repos (spiron, sylphx-ai) use the single hyphenated scale-set name.
   Restore `[self-hosted, sylphx-linux-standard]` to match.

2. Remove vercel.json and the stale README coverage badge that linked to
   a dead *.vercel.app URL. Docs hosting moved to the Sylphx Platform
   (sylphx.toml); Vercel is no longer used. The codecov badge already
   covers the coverage metric.

Note: the Vercel commit-status check itself originates from the Vercel
GitHub App integration and must be disconnected in the Vercel/GitHub App
settings — it can't be removed from the repo alone.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: shtse8

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ on:
 jobs:
   validate:
     name: Validate Code Quality
-    runs-on: [self-hosted, sylphx, linux, standard]
+    runs-on: [self-hosted, sylphx-linux-standard]
     steps:
       - name: Checkout repository
         uses: actions/checkout@v6.0.2

README.md

@@ -8,7 +8,6 @@
 [![License](https://img.shields.io/badge/License-MIT-blue?style=flat-square)](https://opensource.org/licenses/MIT)
 [![CI/CD](https://img.shields.io/github/actions/workflow/status/SylphxAI/pdf-reader-mcp/ci.yml?style=flat-square&label=CI/CD)](https://github.com/SylphxAI/pdf-reader-mcp/actions/workflows/ci.yml)
 [![codecov](https://img.shields.io/codecov/c/github/SylphxAI/pdf-reader-mcp?style=flat-square)](https://codecov.io/gh/SylphxAI/pdf-reader-mcp)
-[![coverage](https://img.shields.io/badge/coverage-94.17%25-brightgreen?style=flat-square)](https://pdf-reader-msu3esos4-sylphx.vercel.app)
 [![TypeScript](https://img.shields.io/badge/TypeScript-5.0-blue.svg?style=flat-square)](https://www.typescriptlang.org/)
 [![Downloads](https://img.shields.io/npm/dm/@sylphx/pdf-reader-mcp?style=flat-square)](https://www.npmjs.com/package/@sylphx/pdf-reader-mcp)