#15242: feat(web-fetch): Add Accept header for Cloudflare Markdown for Agents [AI-assisted]

## Summary This PR implements support for [Cloudflare's Markdown for Agents](https://blog.cloudflare.com/markdown-for-agents/) feature by updating the `Accept` header in `web_fetch`. ## Changes - Changed Accept header from `*/*` to `text/markdown, text/html;q=0.9, */*;q=0.8` - Implements backward-compatible content negotiation using quality values ## How it works The updated Accept header uses HTTP content negotiation with quality values (q=): 1. **Prefers `text/markdown`** (q=1.0 implicit) - Cloudflare will serve optimized markdown if available 2. **Falls back to `text/html`** (q=0.9) - Standard HTML if markdown not available 3. **Accepts any content type** (q=0.8) - Ensures compatibility with all servers This is **fully backward-compatible**: servers that don't support `text/markdown` will continue serving HTML or other formats as before, with no breaking changes. ## Testing - **Degree of testing**: Lightly tested - Verified the change compiles and affects only HTTP request headers - No runtime behavior change for servers that don't support `text/markdown` Fixes #14999 --- ## 🤖 AI Transparency **Generated with**: OpenAI Codex (GPT-5.2) via [OpenClaw](https://github.com/openclaw/openclaw) **Degree of testing**: Lightly tested (compilation verified, single-line change) **Session log**: Available on request **Code understanding confirmed**: ✅ - The change is a single-line modification to the `Accept` header in `src/agents/tools/web-fetch.ts` - HTTP content negotiation with q-values ensures backward compatibility - Servers respond with their preferred format from the Accept list; markdown-capable servers (Cloudflare) will serve markdown, others continue serving HTML