ci: drop the inert GitHub workflows

Gitea ignores `.github/workflows/` entirely once `.gitea/workflows/` is
non-empty, so these two files have had no effect since the release workflow
landed — and the GitHub repo they targeted is gone (404, suspended account),
so they cannot run there either.

They were not merely dead: warm-cache.yml's Monday cron kept firing on the
Gitea side before `.gitea/` existed, queueing jobs against GitHub-hosted
labels (`macos-14`, `windows-2022`, `ubuntu-24.04-arm`) that no self-hosted
runner advertises. Those sat queued forever, and its one dispatchable leg
failed on `rustup: command not found` — the same bare-image gap the Gitea
workflow now provisions around.

`.gitea/workflows/release.yml` is the only pipeline; docs/RELEASE.md already
documents it as such.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/warm-cache.yml

@@ -1,85 +0,0 @@
-name: Warm build cache
-
-# Release builds run on TAG refs, and GitHub Actions cache isolation only
-# lets a run restore caches created on its OWN ref or the DEFAULT branch.
-# No workflow ran on `main`, so every release compiled the whole workspace
-# cold on all five targets (~50 min wall clock, gated by Windows). This
-# workflow builds the same `release-dist` profile on `main` so tag builds
-# restore a warm default-branch cache.
-#
-# Triggers: dependency-affecting pushes to main (each release's version-bump
-# commit warms the cache for the NEXT release), a weekly refresh so the
-# cache never hits GitHub's 7-day unused-eviction, and manual dispatch.
-#
-# The setup steps mirror release.yml's build job (toolchain, target,
-# dotslash/protoc, rust-cache key) — keep them in lockstep, or the cache
-# key won't match and releases go back to cold builds.
-
-on:
-  push:
-    branches: ["main"]
-    paths:
-      - "Cargo.lock"
-      - "Cargo.toml"
-      - "rust-toolchain.toml"
-      - ".github/workflows/warm-cache.yml"
-  schedule:
-    - cron: "17 5 * * 1"
-  workflow_dispatch:
-
-permissions:
-  contents: read
-
-env:
-  CARGO_TERM_COLOR: always
-
-concurrency:
-  group: warm-cache
-  cancel-in-progress: true
-
-jobs:
-  warm:
-    name: warm (${{ matrix.target }})
-    strategy:
-      fail-fast: false
-      matrix:
-        include:
-          - target: aarch64-apple-darwin
-            os: macos-14
-          - target: x86_64-apple-darwin
-            os: macos-14
-          - target: x86_64-unknown-linux-gnu
-            os: ubuntu-24.04
-          - target: aarch64-unknown-linux-gnu
-            os: ubuntu-24.04-arm
-          - target: x86_64-pc-windows-msvc
-            os: windows-2022
-    runs-on: ${{ matrix.os }}
-    steps:
-      - uses: actions/checkout@v4
-
-      - name: Install toolchain (rust-toolchain.toml)
-        run: rustup show
-
-      - name: Add build target
-        run: rustup target add ${{ matrix.target }}
-
-      - name: Install dotslash (protoc launcher)
-        run: cargo install dotslash --locked
-
-      - name: Install protoc (Windows PATH fallback)
-        if: runner.os == 'Windows'
-        shell: pwsh
-        run: |
-          $url = "https://github.com/protocolbuffers/protobuf/releases/download/v29.3/protoc-29.3-win64.zip"
-          Invoke-WebRequest -Uri $url -OutFile protoc.zip
-          Expand-Archive protoc.zip -DestinationPath "$env:USERPROFILE\protoc"
-          Add-Content $env:GITHUB_PATH "$env:USERPROFILE\protoc\bin"
-
-      # Same key as release.yml so tag builds restore this cache verbatim.
-      - uses: Swatinem/rust-cache@v2
-        with:
-          key: ${{ matrix.target }}
-
-      - name: Build kigi (release-dist)
-        run: cargo build --profile release-dist -p kigi-bin --locked --target ${{ matrix.target }}