Remove Dependabot and auto-update workflows

Manual dependency management is simpler and gives more control.

.github/workflows/dependencies.yml

@@ -1,36 +0,0 @@
-name: Dependency Management
-
-on:
-  schedule:
-    - cron: '0 0 * * 1'
-  workflow_dispatch:
-
-permissions:
-  contents: write
-  pull-requests: write
-
-jobs:
-  update-dependencies:
-    name: Update Dependencies
-    runs-on: ubuntu-latest
-    steps:
-      - name: Checkout repository
-        uses: actions/checkout@v4
-
-      - name: Install Rust toolchain
-        uses: dtolnay/rust-toolchain@stable
-
-      - name: Update dependencies
-        run: cargo update
-
-      - name: Create Pull Request
-        uses: peter-evans/create-pull-request@v5
-        with:
-          commit-message: Update Cargo dependencies
-          title: 'chore: update Cargo dependencies'
-          body: |
-            Automated dependency update generated by GitHub Actions.
-
-            Please review the changes and ensure all tests pass before merging.
-          branch: deps/cargo-update
-          delete-branch: true