Add Gitea workflow to sync runner checkout on master changes (#2)

## Summary

Implements #1: Gitea Act workflow that fast-forward pulls `~/Documents/repos/server-infra` on the self-hosted runner whenever `master` changes.

- Adds `.gitea/workflows/sync-checkout.yml`
- Triggers on direct push to `master` and on merged PRs targeting `master`
- Refuses to pull if the working tree is dirty
- Documents the workflow in `IMPLEMENTATION.md`

## Test plan

- [ ] Merge PR → workflow runs on self-hosted runner
- [ ] Runner checkout at `/home/westfarn/Documents/repos/server-infra` advances to latest `master` commit
- [ ] Dirty working tree on runner causes workflow to fail (no silent overwrite)
- [ ] Direct push to `master` also triggers sync

Closes #1

Reviewed-on: #2

.gitea/workflows/sync-checkout.yml

@@ -0,0 +1,36 @@
+name: Sync runner checkout
+
+on:
+  push:
+    branches: [master]
+  pull_request:
+    types: [closed]
+    branches: [master]
+
+jobs:
+  sync:
+    if: gitea.event_name == 'push' || gitea.event.pull_request.merged == true
+    runs-on: self-hosted
+    steps:
+      - name: Pull latest server-infra
+        run: |
+          set -euo pipefail
+          REPO="/home/westfarn/Documents/repos/server-infra"
+
+          if [[ ! -d "${REPO}/.git" ]]; then
+            echo "Missing git checkout at ${REPO}"
+            exit 1
+          fi
+
+          cd "${REPO}"
+
+          if ! git diff --quiet || ! git diff --cached --quiet; then
+            echo "Working tree is dirty; refusing to pull"
+            git status --short
+            exit 1
+          fi
+
+          git fetch origin master
+          git checkout master
+          git pull --ff-only origin master
+          git rev-parse --short HEAD