docs: Add Gitea act_runner setup guide for asgard build server

Architecture clarification:
- Gitea + SeaweedFS run on PUBLIC docker stack
- Act runner runs on ASGARD (Ryzen 9 build server)
- Runner connects remotely to git.corrosionmgmt.com

New documentation:
- ASGARD-RUNNER.md: Complete setup guide for registering
  and running act_runner as systemd service on asgard
- Includes example workflows for companion agent builds
- Troubleshooting and security notes

Runner capabilities:
- Docker access for containerized builds
- Native Go/Rust toolchains (already installed)
- 16C/32T, 64GB DDR5 for fast builds

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

infra/README.md

@@ -21,7 +21,15 @@
 
 ## Deployment
 
-### First-time setup:
+### Architecture Note
+
+**This stack runs on the PUBLIC docker host** (where Nginx Proxy Manager is).
+
+The **Gitea act_runner** runs separately on **asgard** (the build server) and connects to the public Gitea instance remotely.
+
+See `ASGARD-RUNNER.md` for act_runner setup instructions.
+
+### First-time setup (on public docker host):
 ```bash
 cd infra
 docker compose up -d