ci: migrate to L1-uniform thin-caller shape via swift-foundations layer wrapper

Replace .github/workflows/ci.yml with the L1-uniform consumer-caller
pattern (per [CI-031] absolute-minimum shape):

  - ci:   uses swift-foundations/.github/.../swift-ci.yml@main, secrets: inherit
  - docs: uses swift-institute/.github/.../swift-docs.yml@main, secrets: inherit

No with: blocks — DocC umbrella metadata derives per [CI-053]
(swift-{kebab}-{layer} → {Title}_{Layer} → Sources/{Title} {Layer}/...);
enable-private-repos defaults true per [CI-058]; secrets uniform per
[CI-059].

Also delete .github/workflows/swift-format.yml + swiftlint.yml per
[CI-054] (absorbed into the universal swift-ci.yml format and lint
jobs). Per-package .swift-format and .swiftlint.yml config files at
repo root stay per [CI-057].

Provenance: swift-institute/Research/ci-cd-cross-ecosystem-reuse.md
v1.1.1 §Q6 caller fan-out (Phases 5–9); ci-cd-workflows skill
[CI-004a] (Layer Wrappers Mirror Layer Orgs) and [CI-004b] (Sub-Org
Wrappers Are Future Work).

Author: coenttb

.github/workflows/ci.yml

@@ -14,21 +14,13 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
-  # Production validation: Latest Swift on Linux with release build
-  linux-latest:
-    name: Ubuntu (Swift 6.2, release)
-    runs-on: ubuntu-latest
-    container: swift:6.2
-    steps:
-      - uses: actions/checkout@v6
-
-      - name: Cache Swift packages
-        uses: actions/cache@v5
-        with:
-          path: .build
-          key: linux-swift62-spm-v5-${{ hashFiles('Package.swift', 'Package@*.swift') }}
-          restore-keys: linux-swift62-spm-v5-
-
-      - name: Test (release)
-        run: swift test -c release
+  # Build + test matrix — see swift-foundations/.github/.github/workflows/swift-ci.yml (layer wrapper)
+  ci:
+    uses: swift-foundations/.github/.github/workflows/swift-ci.yml@main
+    secrets: inherit
 
+  # DocC umbrella-catalog pipeline per [DOC-019a] — see
+  # swift-institute/.github/.github/workflows/swift-docs.yml
+  docs:
+    uses: swift-institute/.github/.github/workflows/swift-docs.yml@main
+    secrets: inherit