Make global.json global again (#4166)

# Description of Changes

We have had several `global.json` files introduced that aren't symlinks
to the root `global.json`.

This PR fixes that drift, and adds a CI check.

# API and ABI breaking changes

None.

# Expected complexity level and risk

2

# Testing

- [x] `cargo ci global-json-policy` succeeds
- [x] `cargo ci global-json-policy` fails if you add a new `global.json`
file somewhere
- [x] Rest of CI passes, confirming that templates are still working
properly

---------

Signed-off-by: Zeke Foppa <196249+bfops@users.noreply.github.com>
Co-authored-by: Zeke Foppa <bfops@users.noreply.github.com>
Co-authored-by: Copilot Autofix powered by AI <62310815+github-advanced-security[bot]@users.noreply.github.com>

Author: bfops

.github/workflows/ci.yml

@@ -908,3 +908,38 @@ jobs:
               repo: targetRepo,
               run_id: runId,
             });
+
+  global_json_policy:
+    name: Verify global.json files are symlinks
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+    steps:
+      - name: Find Git ref
+        env:
+          PR_NUMBER: ${{ github.event.inputs.pr_number }}
+        run: |
+          if [ -n "$PR_NUMBER" ]; then
+            GIT_REF="refs/pull/$PR_NUMBER/merge"
+          else
+            GIT_REF="${{ github.ref }}"
+          fi
+          echo "GIT_REF=${GIT_REF}" >>"$GITHUB_ENV"
+
+      - name: Checkout sources
+        uses: actions/checkout@v4
+        with:
+          ref: ${{ env.GIT_REF }}
+
+      - uses: dsherret/rust-toolchain-file@v1
+
+      - name: Cache Rust dependencies
+        uses: Swatinem/rust-cache@v2
+        with:
+          workspaces: ${{ github.workspace }}
+          shared-key: spacetimedb
+          save-if: false
+          prefix-key: v1
+
+      - name: Check global.json policy
+        run: cargo ci global-json-policy

sdks/csharp/examples~/regression-tests/republishing/server-initial/global.json

@@ -0,0 +1 @@
+../../../../../../global.json

sdks/csharp/examples~/regression-tests/republishing/server-initial/global.json

@@ -0,0 +1 @@
+../../../../../../global.json

sdks/csharp/examples~/regression-tests/republishing/server-republish/global.json

@@ -0,0 +1 @@
+../../../global.json

sdks/csharp/examples~/regression-tests/republishing/server-republish/global.json

@@ -0,0 +1 @@
+../../../global.json

templates/basic-cs/spacetimedb/global.json

@@ -142,6 +142,17 @@ Usage: self-docs [OPTIONS]
 - `--check`: Only check for changes, do not generate the docs
 - `--help`: Print help (see a summary with '-h')
 
+### `global-json-policy`
+
+**Usage:**
+```bash
+Usage: global-json-policy
+```
+
+**Options:**
+
+- `--help`: 
+
 ### `help`
 
 **Usage:**