Deploy from GitHub with a systemd timer instead of Gitea Actions

Cosmos keeps a deploy-only clone of the GitHub repo. gk-deploy.timer runs
pull-and-deploy.sh every 5 minutes: it fetches master and, when it has
moved, runs deploy.sh (build, check, restart, roll back if unhealthy). A
commit that failed is not retried until master moves (FORCE=1 retries), and
DEPLOY_HEALTHCHECK_URL turns red on a failed fetch or deploy. Removes the
Gitea workflow and updates the runbook, README and inventory note.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012aSbo2D1qP8cMFQCykf7kx

deploy/stack.env.example

@@ -1,5 +1,5 @@
 # Stack settings for compose.yaml. On Cosmos: /home/greg/homelab/gregkilmer/.env
-# The Gitea Actions deploy rewrites GK_TAG on every build; GK_PREVIOUS_TAG is the rollback.
+# deploy.sh rewrites GK_TAG on every deploy; GK_PREVIOUS_TAG is the rollback.
 GK_TAG=
 GK_PREVIOUS_TAG=
 # The vault's Site folder on the host.
@@ -7,3 +7,6 @@ VAULT_SITE=/home/greg/homelab/obsidian-vault/20-29_Makerspace/24_Homelab_Infrast
 # The user the containers run as: the owner of the vault files (id -u greg, id -g greg).
 PUID=1000
 PGID=1000
+# Healthchecks ping for the deploy timer (5-minute period): red when GitHub
+# can't be reached or a commit fails to deploy.
+DEPLOY_HEALTHCHECK_URL=