ci: add uv/docker/github-actions dependabot + run lint/test on dev

- dependabot.yml: add uv ecosystem (updates pyproject.toml + uv.lock
  together), github-actions ecosystem (keeps action versions current),
  and group all Python deps into one weekly PR
- lint.yml, test.yml: extend triggers to dev branch so ruff and pytest
  run on push/PR to dev, not just main

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/lint.yml

@@ -2,9 +2,9 @@ name: Lint Python
 
 on:
   push:
-    branches: ["main"]
+    branches: ["main", "dev"]
   pull_request:
-    branches: ["main"]
+    branches: ["main", "dev"]
 
 jobs:
   lint:

.github/workflows/test.yml

@@ -2,9 +2,9 @@ name: Test Python
 
 on:
   push:
-    branches: ["main"]
+    branches: ["main", "dev"]
   pull_request:
-    branches: ["main"]
+    branches: ["main", "dev"]
 
 jobs:
   test: