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/AGENTS.md

@@ -29,12 +29,12 @@ Context = tokens = cost. Be smart.
 2. Code      → TDD: write test (RED), implement (GREEN), refactor
 3. Test      → pnpm test (must pass)
 4. Push      → git push origin feature/XX-description
-5. PR        → Create PR to develop (not main)
+5. PR        → Create PR to main
 6. Review    → Wait for approval or self-merge if authorized
 7. Close     → Close related issues via API
 ```
 
-**Never merge directly to develop without a PR.**
+**Never merge directly to main without a PR.**
 
 ### Issue Management
 
@@ -53,7 +53,7 @@ curl -s -X PATCH -H "Authorization: token $TOKEN" -H "Content-Type: application/
   -d '{"state":"closed"}'
 
 # Create PR (tea CLI works for this)
-tea pulls create --repo mosaic/stack --base develop --head feature/XX-name \
+tea pulls create --repo mosaic/stack --base main --head feature/XX-name \
   --title "feat(#XX): Title" --description "Description"
 ```