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/CONTRIBUTING.md

@@ -159,13 +159,12 @@ We follow a Git-based workflow with the following branch types:
 
 ### Workflow
 
-1. Always branch from `develop`
-2. Merge back to `develop` via pull request
-3. `main` is for stable releases only
+1. Always branch from `main`
+2. Merge back to `main` via pull request
 
 ```bash
 # Start a new feature
-git checkout develop
+git checkout main
 git pull --rebase
 git checkout -b feature/my-feature-name
 
@@ -269,7 +268,7 @@ Clarified pagination and filtering parameters.
 2. Create a PR via GitLab at:
    https://git.mosaicstack.dev/mosaic/stack/-/merge_requests
 
-3. Target branch: `develop`
+3. Target branch: `main`
 
 4. Fill in the PR template:
    - **Title:** `feat(#issue): Brief description` (follows commit format)