Implements the staging-environments OpenSpec change. Persistent staging at
staging.trails.cool / planner.staging.trails.cool deploys from main; PR
opens get a journal-only preview at pr-<N>.staging.trails.cool that shares
the persistent planner. cd-staging.yml builds tagged images, manages
per-PR Postgres databases and Caddyfile snippets, evicts the oldest
preview at the cap of 3, and tears everything down on PR close.
staging-cleanup.yml runs weekly to sweep orphaned previews.
DNS records (staging + *.staging A/AAAA) already applied to production
via tofu.
Caddy approach: per-PR Caddyfile snippets imported from /etc/caddy/sites/
and reloaded on each PR event — no wildcard / on-demand TLS, no router
service. Production compose gains a trails-shared network for the staging
project to reach Postgres, and host.docker.internal on Caddy so it can
reverse-proxy staging containers published on the host loopback.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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