chore: remove unused GitHub Actions workflow

Woodpecker CI is the active CI system; GitHub Actions never runs on Gitea.

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

.github/workflows/ci.yml

@@ -1,50 +0,0 @@
-name: CI
-
-on:
-  push:
-    branches: [ main, develop ]
-  pull_request:
-    branches: [ main, develop ]
-
-env:
-  CARGO_TERM_COLOR: always
-
-jobs:
-  check:
-    name: Check
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@stable
-      - run: cargo check --all-features
-
-  fmt:
-    name: Format
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@stable
-        with:
-          components: rustfmt
-      - run: cargo fmt --all -- --check
-
-  clippy:
-    name: Clippy
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@stable
-        with:
-          components: clippy
-      - run: cargo clippy --all-features -- -D warnings
-
-  build:
-    name: Build
-    strategy:
-      matrix:
-        os: [ubuntu-latest, macos-latest, windows-latest]
-    runs-on: ${{ matrix.os }}
-    steps:
-      - uses: actions/checkout@v4
-      - uses: dtolnay/rust-toolchain@stable
-      - run: cargo build --release --all-features