docs: correct valhalla OS in the remaining agent-facing docs

Same fix as 317a424, applied to the files an agent actually reads before
touching the host: AGENTS.md and both copies of the homelab-ssh skill.
They all claimed a headless Ubuntu server; valhalla is EndeavourOS (Arch).

Also record two things that cost time this session: there is no apt/apt-get
on the host, and ginnoir has passwordless sudo (needed for anything writing
to root-owned paths like /config/caddy/Caddyfile).

Historical records under plans/ and docs/superpowers/plans/ are left alone
on purpose — they document what was true when the work was done.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

AGENTS.md

@@ -9,9 +9,9 @@ from your tool's user-level instructions (`~/.codex/AGENTS.md` / `~/.gemini/GEMI
 
 ## Essentials (full detail in `CLAUDE.md`)
 
-- **What this is:** deployment config for a live, single-host Docker homelab on a headless Ubuntu
-  server reachable at `ssh ginnoir@valhalla`. ~50 containers in per-domain Portainer-managed stacks.
-  This is **not** application code.
+- **What this is:** deployment config for a live, single-host Docker homelab on a headless
+  **EndeavourOS (Arch)** server reachable at `ssh ginnoir@valhalla` — `pacman`, not `apt`.
+  ~50 containers in per-domain Portainer-managed stacks. This is **not** application code.
 - **The repo is canonical.** Portainer polls `main` every 5 min and redeploys any app stack whose
   `stacks/<domain>/*` files changed. Editing here changes nothing until you `git push`.
 - **Deployment channels:**