feat(proxy): Markdown for Agents — Accept: text/markdown content negotiation

[dviejokfs]

Summary

Implements Markdown for Agents content negotiation in the Pingora reverse proxy, compatible with Cloudflare's emerging standard for AI-native content delivery.

When an AI agent or HTTP client sends Accept: text/markdown, Temps now converts the upstream HTML response to Markdown on the fly before delivering it — reducing token waste and improving response quality for AI systems consuming deployed apps.

How it works

curl https://your-app.temps.sh/ -H "Accept: text/markdown"
# → Content-Type: text/markdown; charset=utf-8
# → Vary: Accept
# → X-Markdown-Tokens: <estimated count>
# → [Markdown body]

Filter chain

Stage	What happens
early_request_filter	Detects Accept: text/markdown; sets ctx.wants_markdown = true; disables upstream compression
upstream_response_filter	Confirms upstream is text/html (not SSE/WS); adds Vary: Accept; cancels conversion otherwise
response_filter	Rewrites Content-Type → text/markdown; charset=utf-8; removes Content-Length and Content-Encoding; adds X-Markdown-Tokens: 0 placeholder
response_body_filter	Buffers chunks; converts HTML → Markdown via htmd on end_of_stream; falls back to passthrough on error or size limit exceeded

Safety guards

• SSE/WebSocket: conversion is cancelled if ctx.is_sse or ctx.is_websocket is set — streaming responses always pass through unchanged
• Non-HTML: only text/html upstream responses are converted; JSON, images, etc. pass through as-is
• 2 MB limit: responses larger than 2 MB fall back to passthrough, mirroring Cloudflare's own constraint
• Conversion failure: if htmd fails, the original HTML bytes are returned so the client always gets something

Tests

18 unit tests added in proxy::markdown_tests:

• Accept header parsing (exact, quality values, case-insensitive, negative)
• Content-type gating (HTML converts, JSON/SSE/WS do not)
• Multi-chunk body accumulation
• Size guard (>2 MB disables conversion)
• HTML→Markdown conversion correctness
• SSE passthrough safety
• Token estimation

Dependencies

• htmd 0.5 — pure Rust HTML→Markdown converter, no C dependencies