chore: rename stack .env files to stack.env for Portainer git stack compatibility

Portainer git-stack deploys require stack.env (not .env) in the repo.
Renames all stacks/*/. env → stacks/*/stack.env so Portainer reads
secrets directly from git on each redeploy, making the repo the single
source of truth instead of requiring manual UI sync.

Also fixes the dev stack BOOKSTACK_APP_KEY gap — the key was already
present in the file but missing from Portainer's stored envVars; it
will now be picked up automatically from stack.env on next redeploy.

Updates CLAUDE.md to reflect the new filename convention.

stacks/dev/stack.env

@@ -0,0 +1,21 @@
+# dev stack secrets
+TZ=America/Chicago
+
+# Gitea
+GITEA_DB_PASSWORD=gitea_d3vp4ss_9f2k
+
+# code-server (VS Code in browser)
+CODE_SERVER_PASSWORD=c0de_s3rver_pass
+
+# BookStack wiki
+# APP_KEY encrypts session data — DO NOT rotate without invalidating sessions.
+BOOKSTACK_APP_KEY=base64:cSR6L4TPEbM2IFi9MyZcnmexvzAlIHAjn/qMzQKO/t4=
+BOOKSTACK_DB_PASSWORD=bookstack_d3v_pass
+BOOKSTACK_DB_ROOT_PASSWORD=bookstack_r00t_pass
+
+# Plane project management
+# Rotate with: openssl rand -hex 32
+PLANE_SECRET_KEY=60gp0byfz2dvffa45cxl20p1scy9xbpf6d8c5y0geejgkyp1b52
+PLANE_DB_PASSWORD=plane_d3v_p4ss_8x3m
+PLANE_MINIO_USER=planeadmin
+PLANE_MINIO_PASSWORD=plane_minio_p4ss