docs: standardize configuration examples to use singular 'replica' format

Update all documentation, examples, and test helpers to use the
singular `replica:` format instead of the deprecated `replicas:` array
format. This aligns with:
- Current codebase architecture (one replica per database)
- Online documentation standard at litestream.io
- Deprecation of plural format in cmd/litestream/main.go:478-479

Changes:
- etc/litestream.yml: Convert to singular format (from PR #749)
- .claude/agents/replica-client-developer.md: Update config pattern
- .claude/commands/add-storage-backend.md: Update documentation example
- .github/workflows/commit.yml: Update CI test configs
- AGENTS.md: Update configuration example
- tests/integration/soak_helpers.go: Update test config generator

The code continues to support both formats for backward compatibility,
but the singular format is now the documented standard.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: corylanou

.claude/agents/replica-client-developer.md

@@ -101,10 +101,10 @@ go test -race -v ./[backend]/...
 ## Configuration Pattern
 
 ```yaml
-replicas:
-  - type: [backend]
-    option1: value1
-    option2: value2
+replica:
+  type: [backend]
+  option1: value1
+  option2: value2
 ```
 
 ## References

.claude/commands/add-storage-backend.md

@@ -71,10 +71,10 @@ Create a new storage backend implementation for Litestream with all required com
 5. **Add Documentation**
    Update README.md with configuration example:
    ```yaml
-   replicas:
-     - type: {{backend_name}}
-       option1: value1
-       option2: value2
+   replica:
+     type: {{backend_name}}
+     option1: value1
+     option2: value2
    ```
 
 ## Key Requirements

.github/workflows/commit.yml

@@ -300,13 +300,13 @@ jobs:
           cat > /tmp/sftp-concurrent.yml <<EOF
           dbs:
             - path: /tmp/test-concurrent.db
-              replicas:
-                - type: sftp
-                  host: localhost:2222
-                  key-path: /test/id_ed25519
-                  user: root
-                  path: /test/data/concurrent
-                  concurrent-writes: true
+              replica:
+                type: sftp
+                host: localhost:2222
+                key-path: /test/id_ed25519
+                user: root
+                path: /test/data/concurrent
+                concurrent-writes: true
           EOF
 
           # Create test database
@@ -326,12 +326,12 @@ jobs:
           cat > /tmp/sftp-sequential.yml <<EOF
           dbs:
             - path: /tmp/test-sequential.db
-              replicas:
-                - type: sftp
-                  host: localhost:2222
-                  key-path: /test/id_ed25519
-                  user: root
-                  path: /test/data/sequential
+              replica:
+                type: sftp
+                host: localhost:2222
+                key-path: /test/id_ed25519
+                user: root
+                path: /test/data/sequential
                   concurrent-writes: false
           EOF
 

AGENTS.md

@@ -771,11 +771,11 @@ go test -race -v -run TestStore_CompactDB ./...
 ```yaml
 dbs:
   - path: /path/to/db.sqlite
-    replicas:
-      - type: s3
-        bucket: my-bucket
-        path: db-backup
-        sync-interval: 10s  # How often to sync
+    replica:
+      type: s3
+      bucket: my-bucket
+      path: db-backup
+      sync-interval: 10s  # How often to sync
 
 # Compaction configuration (default)
 levels:

tests/integration/soak_helpers.go

@@ -418,8 +418,8 @@ func CreateSoakConfig(dbPath, replicaURL string, s3Config *S3Config, shortMode b
 	config.WriteString("    min-checkpoint-page-count: 100\n")
 	config.WriteString("    max-checkpoint-page-count: 5000\n")
 	config.WriteString("\n")
-	config.WriteString("    replicas:\n")
-	config.WriteString(fmt.Sprintf("      - url: %s\n", replicaURL))
+	config.WriteString("    replica:\n")
+	config.WriteString(fmt.Sprintf("      url: %s\n", replicaURL))
 
 	// Add S3-specific settings if provided
 	if s3Config != nil {