Remove ghp agent role (missing installation_id; unblock apply)

The ghp agent role fails at apply with `installation_id is required for
agent tokens` because config/ghp_secret_backend_role/ghp/agent.yaml carries a
placeholder installation_id. Remove the role for now so the master apply goes
green and does not block the arrstack #127 apply. The ghp backend is retained;
re-add the role once a real installation_id is available.

- Delete config/ghp_secret_backend_role/ghp/agent.yaml (empties the
  ghp_secret_backend_role for_each map).

config/ghp_secret_backend_role/ghp/agent.yaml

@@ -1,15 +0,0 @@
-# Role minting ephemeral, scoped ghp agent tokens. Reading ghp/creds/agent mints
-# a lease-bound token deleted from ghp on revoke/expiry. token_type "agent" binds
-# the minted token to a ghp App installation, so installation_id is REQUIRED.
-#
-# installation_id below is a PLACEHOLDER (0) and MUST be set to the real ghp App
-# installation id before this role can mint usable tokens. scopes are ghp
-# permission:level pairs; contents:read is the least-privilege default.
----
-token_type: agent
-installation_id: 0 # PLACEHOLDER - set to the real ghp App installation id
-scopes:
-  - contents:read
-session_prefix: vault
-ttl: 3600      # 1h
-max_ttl: 86400 # 24h