ci: split CI into reusable workflows (#7168)

## Why

The monolithic CI workflow is a frequent merge-conflict hotspot.
Splitting cohesive domains into same-repository reusable workflows keeps
one centrally filtered entry point while letting Rust, desktop,
relay/PostgreSQL, client, and security CI evolve independently.

## What

- Keep `ci.yml` as the only push/pull-request orchestrator with
unchanged concurrency and path detection.
- Move 18 execution jobs into five `workflow_call`-only domain workflows
without changing their runners, steps, matrices, caches, artifacts,
permissions, or timeouts.
- Keep the relay artifact producer with desktop integration, the
complete PostgreSQL lane, and relay E2E consumers.
- Preserve all 12 existing required GitHub Actions contexts through
lightweight top-level compatibility gates, so the repository ruleset
does not need to change.
- Update the Rust-cache contract to follow Unit Tests into
`_ci-rust.yml`.

## Risk Assessment

CI-only change with moderate workflow-orchestration risk. The main risks
are reusable-workflow output propagation, skip behavior, and visible
check naming; the old required names remain explicit top-level jobs, and
the draft will stay open until an exact-head GitHub Actions run and
independent review are complete.

Generated with Codex

---------

Signed-off-by: Luke Tornquist <tornquist@squareup.com>
Signed-off-by: tornquist <tornquist@squareup.com>

.github/workflows/_ci-clients.yml

@@ -0,0 +1,148 @@
+name: CI / Clients
+on:
+  workflow_call:
+    inputs:
+      web:
+        required: true
+        type: boolean
+      mobile:
+        required: true
+        type: boolean
+      lane:
+        required: true
+        type: string
+    outputs:
+      web_result:
+        value: ${{ jobs.results.outputs.web_result }}
+      mobile_result:
+        value: ${{ jobs.results.outputs.mobile_result }}
+
+env:
+  CARGO_TERM_COLOR: always
+  BUZZ_TEST_POSTGRES_PASSWORD: buzz_dev
+  PLAYWRIGHT_BROWSERS_PATH: ${{ github.workspace }}/.cache/ms-playwright
+
+jobs:
+  web:
+    name: Web
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    if: inputs.lane == 'required' && (github.event_name == 'push' || inputs.web)
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+        with:
+          fetch-depth: 2
+      - uses: cashapp/activate-hermit@cea9af7913204a965fd488637a8d1811bba2e616 # v1
+      - name: Get pnpm store directory
+        id: pnpm-cache
+        run: echo "STORE_PATH=$(pnpm store path --silent)" >> "$GITHUB_OUTPUT"
+      - name: Restore pnpm store cache
+        uses: actions/cache/restore@caa296126883cff596d87d8935842f9db880ef25 # v5
+        with:
+          path: ${{ steps.pnpm-cache.outputs.STORE_PATH }}
+          key: pnpm-${{ runner.os }}-${{ hashFiles('**/pnpm-lock.yaml') }}
+          restore-keys: pnpm-${{ runner.os }}-
+      - name: Install dependencies
+        run: pnpm install --frozen-lockfile
+      - name: Web lint and format
+        run: just web-check
+      - name: Web build
+        run: just web-build
+      - name: Save pnpm store cache
+        if: github.event_name == 'push'
+        uses: actions/cache/save@caa296126883cff596d87d8935842f9db880ef25 # v5
+        with:
+          path: ${{ steps.pnpm-cache.outputs.STORE_PATH }}
+          key: pnpm-${{ runner.os }}-${{ hashFiles('**/pnpm-lock.yaml') }}
+
+  mobile:
+    name: Mobile
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    if: inputs.lane == 'required' && (github.event_name == 'push' || inputs.mobile)
+    permissions:
+      contents: read
+    steps:
+      - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+        with:
+          fetch-depth: 2
+      - uses: cashapp/activate-hermit@cea9af7913204a965fd488637a8d1811bba2e616 # v1
+      - name: Compute Hermit cache key
+        id: hermit-bin-hash
+        run: |
+          hash="$(find ./bin ! -type d | sort | xargs openssl sha256 | openssl sha256 -r | cut -d' ' -f1)"
+          echo "hash=$hash" >> "$GITHUB_OUTPUT"
+      - name: Restore Hermit package cache
+        id: hermit-cache
+        uses: actions/cache/restore@caa296126883cff596d87d8935842f9db880ef25 # v5
+        with:
+          path: ~/.cache/hermit/pkg
+          key: ${{ runner.os }}-hermit-cache-${{ steps.hermit-bin-hash.outputs.hash }}
+          restore-keys: ${{ runner.os }}-hermit-cache-
+      - name: Prime Flutter SDK
+        run: flutter --version
+      - name: Save Hermit package cache
+        if: always() && steps.hermit-cache.outputs.cache-hit != 'true'
+        uses: actions/cache/save@caa296126883cff596d87d8935842f9db880ef25 # v5
+        continue-on-error: true
+        with:
+          path: ~/.cache/hermit/pkg
+          key: ${{ runner.os }}-hermit-cache-${{ steps.hermit-bin-hash.outputs.hash }}
+      - name: Restore pub cache
+        id: pub-cache
+        uses: actions/cache/restore@caa296126883cff596d87d8935842f9db880ef25 # v5
+        with:
+          path: ~/.pub-cache
+          key: pub-${{ runner.os }}-${{ hashFiles('mobile/pubspec.lock') }}
+          restore-keys: pub-${{ runner.os }}-
+      - name: Install dependencies
+        run: cd mobile && flutter pub get
+      - name: Save pub cache
+        if: always() && steps.pub-cache.outputs.cache-hit != 'true'
+        uses: actions/cache/save@caa296126883cff596d87d8935842f9db880ef25 # v5
+        continue-on-error: true
+        with:
+          path: ~/.pub-cache
+          key: pub-${{ runner.os }}-${{ hashFiles('mobile/pubspec.lock') }}
+      - name: Format check
+        run: cd mobile && dart format --output=none --set-exit-if-changed .
+      - name: Analyze
+        run: cd mobile && flutter analyze
+      - name: Test
+        run: cd mobile && flutter test
+      - name: Build Android debug APK
+        run: just mobile-build-android
+
+  mobile-swift:
+    name: Mobile Swift
+    runs-on: macos-latest
+    timeout-minutes: 30
+    if: inputs.lane == 'mobile-swift' && inputs.mobile
+    steps:
+      - uses: actions/checkout@df4cb1c069e1874edd31b4311f1884172cec0e10 # v6.0.3
+      - uses: cashapp/activate-hermit@cea9af7913204a965fd488637a8d1811bba2e616 # v1
+      - name: Install Flutter dependencies
+        run: cd mobile && flutter pub get
+      - name: Build
+        run: swift build --package-path mobile/ios/BuzzPushKit
+      - name: Build release
+        run: swift build -c release --package-path mobile/ios/BuzzPushKit
+      - name: Test
+        run: swift test --package-path mobile/ios/BuzzPushKit
+      - name: Build complete unsigned iOS release
+        run: cd mobile && flutter build ios --release --no-codesign --no-pub
+
+  results:
+    name: Results
+    if: ${{ always() }}
+    needs: [web, mobile, mobile-swift]
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    permissions: {}
+    outputs:
+      web_result: ${{ needs.web.result }}
+      mobile_result: ${{ needs.mobile.result }}
+    steps:
+      - run: echo "Captured client job results"