feat(helm): migrate to new environment variable naming convention

Replace deprecated environment variables with new consolidated names:
- VECTOR_SYNC_ENABLED → ENABLE_SEMANTIC_SEARCH
- ENABLE_OFFLINE_ACCESS → ENABLE_BACKGROUND_OPERATIONS

Update values.yaml structure:
- Rename 'vectorSync' section to 'semanticSearch'
- Update descriptions to emphasize BM25 hybrid search

Benefits:
- Aligns with application-level config consolidation
- Clearer naming: "semantic search" vs "vector sync"
- Maintains backward compatibility via application deprecation handling
- Automatic enablement of background ops when semantic search enabled in multi-user modes

Updated files:
- values.yaml: Renamed vectorSync → semanticSearch
- deployment.yaml: New env var names with deprecation comments
- NOTES.txt: Updated deployment notes
- README.md: Updated documentation and examples

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

charts/nextcloud-mcp-server/values.yaml

@@ -358,10 +358,11 @@ extraEnvFrom: []
 # - secretRef:
 #     name: my-secret
 
-# Vector Sync Configuration
-# Background synchronization of Nextcloud content into vector database for semantic search
-vectorSync:
-  # Enable background vector synchronization
+# Semantic Search Configuration
+# Enable semantic search with BM25 hybrid search and background synchronization
+# of Nextcloud content into vector database
+semanticSearch:
+  # Enable semantic search and background vector synchronization
   enabled: false
   # Scan interval in seconds (how often to check for changes)
   scanInterval: 3600
@@ -372,7 +373,7 @@ vectorSync:
 
 # Document Chunking Configuration
 # Controls how documents are split into chunks before embedding
-# Only relevant when vectorSync.enabled is true
+# Only relevant when semanticSearch.enabled is true
 documentChunking:
   # Number of words per chunk (default: 512)
   # Smaller chunks (256-384): Better for precise searches, more chunks to store