chore: switch from develop/dev to main/latest image tags

Remove develop branch references from CI, compose, env, and docs
now that all development uses trunk-based workflow on main.

- CI: remove develop branch filters and dev tag logic
- Compose: default IMAGE_TAG from dev to latest
- Env: update IMAGE_TAG default and comments
- Docs: update branching strategy, PR targets, and image tag docs

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

docker-compose.openbao.yml

@@ -14,7 +14,7 @@ services:
   # OpenBao Secrets Vault
   # ======================
   openbao:
-    image: git.mosaicstack.dev/mosaic/stack-openbao:${IMAGE_TAG:-dev}
+    image: git.mosaicstack.dev/mosaic/stack-openbao:${IMAGE_TAG:-latest}
     entrypoint: ["dumb-init", "--"]
     command: ["bao", "server", "-config=/openbao/config/config.hcl"]
     environment:
@@ -48,7 +48,7 @@ services:
   # Has built-in retry logic (polls OpenBao API for 60 seconds).
   # After init, runs an unseal watch loop to handle container restarts.
   openbao-init:
-    image: git.mosaicstack.dev/mosaic/stack-openbao:${IMAGE_TAG:-dev}
+    image: git.mosaicstack.dev/mosaic/stack-openbao:${IMAGE_TAG:-latest}
     command: /openbao/init.sh
     environment:
       VAULT_ADDR: http://openbao:8200