* Add systemd_exporter Ansible role and Prometheus scrape config
- Create systemd_exporter role (download binary, create user, deploy service)
- Add scrape job for london-b:9558 and copenhagen-a:9558
- Add systemd_exporter_hosts inventory group
- Add stage 3b to deploy.yml
- Map role to deploy-on-merge scope
Closes PESO-120
* Fix line length lint violations in systemd_exporter tasks
* Fix var-naming lint: use systemd_exporter_ prefix for role variables
Instead of deploying to the entire fleet on every merge, detect which
files changed and limit ansible-playbook to only affected hosts.
Maps ansible roles, services, and host_vars to their target hosts.
Falls back to full fleet deploy for unmapped paths or changes to
shared infrastructure (common role, deploy.yml, inventory).
Closes PESO-108
Both deploy-on-merge.yml and deploy.yml install ansible via pip but
never install the required Galaxy collections (community.docker,
community.general, ansible.posix) from ansible/requirements.yml.
This works by accident because the pip ansible package bundles some
collections, but it's fragile — a pip upgrade or runner image change
could break deploys silently.
Fixes PESO-110
