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.yml

@@ -3,7 +3,7 @@ services:
   # PostgreSQL Database
   # ======================
   postgres:
-    image: git.mosaicstack.dev/mosaic/stack-postgres:${IMAGE_TAG:-dev}
+    image: git.mosaicstack.dev/mosaic/stack-postgres:${IMAGE_TAG:-latest}
     container_name: mosaic-postgres
     restart: unless-stopped
     environment:
@@ -251,7 +251,7 @@ services:
   # OpenBao Secrets Management (Optional)
   # ======================
   openbao:
-    image: git.mosaicstack.dev/mosaic/stack-openbao:${IMAGE_TAG:-dev}
+    image: git.mosaicstack.dev/mosaic/stack-openbao:${IMAGE_TAG:-latest}
     container_name: mosaic-openbao
     restart: unless-stopped
     user: root
@@ -283,7 +283,7 @@ services:
       - "com.mosaic.description=OpenBao secrets management"
 
   openbao-init:
-    image: git.mosaicstack.dev/mosaic/stack-openbao:${IMAGE_TAG:-dev}
+    image: git.mosaicstack.dev/mosaic/stack-openbao:${IMAGE_TAG:-latest}
     container_name: mosaic-openbao-init
     restart: unless-stopped
     user: root
@@ -345,7 +345,7 @@ services:
   # Mosaic API
   # ======================
   api:
-    image: git.mosaicstack.dev/mosaic/stack-api:${IMAGE_TAG:-dev}
+    image: git.mosaicstack.dev/mosaic/stack-api:${IMAGE_TAG:-latest}
     container_name: mosaic-api
     restart: unless-stopped
     environment:
@@ -424,7 +424,7 @@ services:
   # Mosaic Orchestrator
   # ======================
   orchestrator:
-    image: git.mosaicstack.dev/mosaic/stack-orchestrator:${IMAGE_TAG:-dev}
+    image: git.mosaicstack.dev/mosaic/stack-orchestrator:${IMAGE_TAG:-latest}
     container_name: mosaic-orchestrator
     restart: unless-stopped
     # Run as non-root user (node:node, UID 1000)
@@ -491,7 +491,7 @@ services:
   # Mosaic Web
   # ======================
   web:
-    image: git.mosaicstack.dev/mosaic/stack-web:${IMAGE_TAG:-dev}
+    image: git.mosaicstack.dev/mosaic/stack-web:${IMAGE_TAG:-latest}
     container_name: mosaic-web
     restart: unless-stopped
     environment: