fix(harnesses): scoped account_ref for ambiguous gitea-* providers

gitea-ssh has 3 accounts (ai_enablement, accelerators, homelab) and
gitea-https has 2 (primary, admin) — bare provider_type refs hit
MultipleAccountsError per SR-DISP-8. Switch to scoped name.space:platform
form. Mirrors the agent-runtimes commit; the dispatcher-side parser
also gains underscore support for account names like 'ai_enablement'.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

harnesses/contexts/gitea-admin/v1/harness.yaml

@@ -19,7 +19,7 @@ ssh_hosts:
 secrets_required:
   # ESO-managed gitea-admin token (M22 Phase 9)
   - name: gitea-admin
-    account_ref: "gitea-admin"
+    account_ref: "admin.space:platform"
     mount_path: /run/agent/secrets/gitea-admin
     mode: "0400"
   # Legacy SSH key (file-based, already delivered as mount)