ci: switch all GitHub-hosted runners to self-hosted

Replaces ubuntu-latest and macos-26 with self-hosted to eliminate
GitHub Actions billing. Affects ci.yml, release.yml, claude.yml, claude-code-review.yml.

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

Author: Stijn Willems

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ jobs:
       matrix:
         xcode: ['26.2']
         config: ['debug', 'release']
-    runs-on: macos-26
+    runs-on: self-hosted
     steps:
       - uses: actions/checkout@v5
       - name: Select Xcode ${{ matrix.xcode }}
@@ -31,7 +31,7 @@ jobs:
         xcode: ['26.2']
         config: ['debug']
         scheme: ['Reminders', 'CaseStudies', 'SyncUps']
-    runs-on: macos-26
+    runs-on: self-hosted
     continue-on-error: true
     steps:
       - uses: actions/checkout@v5
@@ -60,7 +60,7 @@ jobs:
   #     matrix:
   #       swift:
   #         - '6.1'
-  #   runs-on: ubuntu-latest
+  #   runs-on: self-hosted
   #   container: swift:${{ matrix.swift }}
   #   steps:
   #     - uses: actions/checkout@v4

.github/workflows/claude-code-review.yml

@@ -18,7 +18,7 @@ jobs:
     #   github.event.pull_request.user.login == 'new-developer' ||
     #   github.event.pull_request.author_association == 'FIRST_TIME_CONTRIBUTOR'
 
-    runs-on: ubuntu-latest
+    runs-on: self-hosted
     permissions:
       contents: read
       pull-requests: read

.github/workflows/claude.yml

@@ -17,7 +17,7 @@ jobs:
       (github.event_name == 'pull_request_review_comment' && contains(github.event.comment.body, '@claude')) ||
       (github.event_name == 'pull_request_review' && contains(github.event.review.body, '@claude')) ||
       (github.event_name == 'issues' && (contains(github.event.issue.body, '@claude') || contains(github.event.issue.title, '@claude')))
-    runs-on: ubuntu-latest
+    runs-on: self-hosted
     permissions:
       contents: read
       pull-requests: read

.github/workflows/release.yml

@@ -5,7 +5,7 @@ on:
   workflow_dispatch:
 jobs:
   project-channel:
-    runs-on: ubuntu-latest
+    runs-on: self-hosted
     steps:
       - name: Dump Github context
         env:
@@ -44,7 +44,7 @@ jobs:
             ]
 
   releases-channel:
-    runs-on: ubuntu-latest
+    runs-on: self-hosted
     steps:
       - name: Dump Github context
         env: