AGENTS.md: add huggingface to the Assisted-by api-location table

huggingface.co is a public provider endpoint (acts as a proxy to backing
providers). Add it so the Assisted-by trailer api-location derivation
resolves the huggingface provider prefix to huggingface.co instead of
unknown.

Assisted-by: harness=pi; primary-model=huggingface/zai-org/GLM-5.2; api-location=huggingface.co

AGENTS.md

@@ -135,6 +135,7 @@ These conventions apply to every VCS commit an agent makes - git commits and fos
   | `groq` | `groq.com` |
   | `together` | `together.ai` |
   | `deepseek` | `deepseek.com` |
+  | `huggingface` | `huggingface.co` |
   | `x-ai` / `xai` | `x.ai` |
 - `secondary-models` is intentionally omitted (not reliably introspectable). May be added later if a harness surfaces subagent model info.
 - The `Assisted-by` trailer describes tooling, not authorship; it does not replace or conflict with the `Co-Authored-By` ban above.