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>

.woodpecker/README.md

@@ -85,12 +85,11 @@ install -> [ruff-check, mypy, security-bandit, security-pip-audit, test]
 
 ## Image Tagging
 
-| Condition        | Tag                        | Purpose                    |
-| ---------------- | -------------------------- | -------------------------- |
-| Always           | `${CI_COMMIT_SHA:0:8}`     | Immutable commit reference |
-| `main` branch    | `latest`                   | Current production release |
-| `develop` branch | `dev`                      | Current development build  |
-| Git tag          | tag value (e.g., `v1.0.0`) | Semantic version release   |
+| Condition     | Tag                        | Purpose                    |
+| ------------- | -------------------------- | -------------------------- |
+| Always        | `${CI_COMMIT_SHA:0:8}`     | Immutable commit reference |
+| `main` branch | `latest`                   | Current latest build       |
+| Git tag       | tag value (e.g., `v1.0.0`) | Semantic version release   |
 
 ## Required Secrets
 
@@ -138,5 +137,5 @@ Fails on blockers or critical/high severity security findings.
 
 ### Pipeline runs Docker builds on pull requests
 
-- Docker build steps have `when: branch: [main, develop]` guards
+- Docker build steps have `when: branch: [main]` guards
 - PRs only run quality gates, not Docker builds