feat(custodian-agent): Ansible role + Makefile for Custodian SSH identity

Establishes a dedicated SSH keypair for the Custodian automation agent:
- ansible/roles/custodian_agent/: authorized_key task (tagged custodian_agent)
- ansible/inventory/group_vars/all.yaml: custodian_agent_user/pubkey vars
- ansible/playbooks/bootstrap.yaml: custodian_agent role added
- Makefile: provision-custodian-agent / provision-custodian-agent-host targets

Keypair generation: cd ~/the-custodian && make custodian-keygen
Then deploy:        cd ~/railiance-infra && make provision-custodian-agent

The private key lives at ~/.ssh/id_custodian_agent — never committed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

ansible/playbooks/bootstrap.yaml

@@ -6,4 +6,5 @@
   roles:
     - role: base
     - role: sops_agent
+    - role: custodian_agent   # injects ~/.ssh/id_custodian_agent.pub into authorized_keys
     # - role: wireguard   # enable if you configure WireGuard variables