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>

docs/PORTAINER-DEPLOYMENT.md

@@ -62,7 +62,7 @@ If using private registry images from `git.mosaicstack.dev`:
 4. **Web editor:** Copy and paste contents of `docker-compose.portainer.yml`
 5. **Environment variables:**
    ```
-   IMAGE_TAG=dev
+   IMAGE_TAG=latest
    OPENBAO_PORT=8200
    ```
 6. Click **Deploy the stack**
@@ -90,7 +90,7 @@ If using private registry images from `git.mosaicstack.dev`:
 **Option A: Git Repository (Recommended)**
 
 - Repository URL: `https://git.mosaicstack.dev/mosaic/stack`
-- Repository reference: `refs/heads/develop`
+- Repository reference: `refs/heads/main`
 - Compose path: `docker-compose.swarm.yml`
 - Authentication: Enable if repository is private
 - Enable **Automatic updates** (optional)
@@ -103,7 +103,7 @@ If using private registry images from `git.mosaicstack.dev`:
 4. **Environment variables:**
 
    ```
-   IMAGE_TAG=dev
+   IMAGE_TAG=latest
    POSTGRES_PASSWORD=<your-secure-password>
    JWT_SECRET=<your-jwt-secret>
    BETTER_AUTH_SECRET=<your-auth-secret>
@@ -148,7 +148,7 @@ If using private registry images from `git.mosaicstack.dev`:
 
 ```bash
 # Image Configuration
-IMAGE_TAG=dev                    # or 'latest' or specific commit SHA
+IMAGE_TAG=latest                    # or 'latest' or specific commit SHA
 
 # Database
 POSTGRES_PASSWORD=<secure-password>