remove hosts/ directory — single source of truth is inventory.yaml

- Delete cmd/oikos/build_hosts.go (generator no longer needed)
- Remove build-hosts subcommand from main.go
- Fix oikos homelab whoami: read from inventory.yaml instead of hosts/
- Update bootstrap.sh: identity check uses inventory.yaml
- Remove all 27 generated hosts/*.yaml files
- Update AGENTS.md and OIKOS.md to reference inventory.yaml only

.agents/OIKOS.md

@@ -38,7 +38,7 @@ one pass through **Observe → Orient → Decide → Act**:
 
 | Primitive | What it is | Lives in |
 |---|---|---|
-| Host / Service | topology entities | `inventory.yaml` (+ generated `hosts/*.yaml`) |
+| Host / Service | topology entities | `inventory.yaml` |
 | Secret | SOPS+age encrypted value, per-client recipients | `secrets/` + `.sops.yaml` |
 | Runbook | executable workflow with risk class + verification | `.agents/skills/<name>/SKILL.md` |
 | Signal | something needing attention, with lifecycle | `signals/` ledger (Week 3) |