Harden CI permissions and pin actions; add Dependabot config

.github/dependabot.yml

@@ -0,0 +1,25 @@
+version: 2
+updates:
+  # Actions are SHA-pinned in the workflows, so this is what moves those pins.
+  # Without it, a pinned action never receives a security fix.
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: "ci"
+    groups:
+      actions:
+        patterns: ["*"]
+
+  # CodeSnake has no runtime dependencies; this covers the optional [tools]
+  # and [dev] extras in pyproject.toml.
+  - package-ecosystem: "pip"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    commit-message:
+      prefix: "deps"
+    groups:
+      tools:
+        patterns: ["*"]

.github/workflows/ci.yml

@@ -5,16 +5,26 @@ on:
     branches: [main]
   pull_request:
 
+# Deny by default; each job grants only what it needs.
+permissions: {}
+
 jobs:
   test:
     runs-on: ubuntu-latest
+    permissions:
+      contents: read
     strategy:
       fail-fast: false
       matrix:
         python-version: ["3.10", "3.11", "3.12", "3.13"]
     steps:
-      - uses: actions/checkout@v7
-      - uses: actions/setup-python@v7
+      # Actions are pinned to a commit SHA: a tag is a mutable ref, and whoever
+      # can move it can run arbitrary code in this workflow.
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          # Do not leave the job token in .git/config for later steps to reuse.
+          persist-credentials: false
+      - uses: actions/setup-python@5fda3b95a4ea91299a34e894583c3862153e4b97 # v7.0.0
         with:
           python-version: ${{ matrix.python-version }}
       - name: Install