feat(http): accept text/markdown

[robherley]

so the agents can read snips 🤖

$ curl http://localhost:8080/f/p2XHxsuB3z -H "Accept: text/markdown"

---
id: p2XHxsuB3z
size: 1.7 kB
type: go
created: 2026-01-25T00:33:06Z
updated: 2026-01-25T00:33:06Z
source: http://localhost:8080/f/p2XHxsuB3z
---

```go
package main

import (
	"context"

...