feat(helm): Add document chunking configuration

Add support for configurable document chunking parameters to Helm chart
to match docker-compose and application capabilities.

Changes:
1. values.yaml:
   - Add documentChunking section with chunkSize (512) and chunkOverlap (50)
   - Include comprehensive comments explaining chunking strategies
   - Positioned between vectorSync and qdrant sections

2. templates/deployment.yaml:
   - Add DOCUMENT_CHUNK_SIZE and DOCUMENT_CHUNK_OVERLAP env vars
   - Always set (not conditional), used by vector sync processor
   - Environment variables follow same pattern as config.py defaults

3. README.md:
   - Add documentChunking parameter table in Vector Search section
   - Document chunking strategies (small/medium/large chunks)
   - Explain overlap recommendations (10-20% of chunk size)

Validation:
- helm lint: Passes
- helm template: Environment variables correctly generated
- Custom values: Work as expected (tested with chunkSize=1024)
- Always present: Not conditional on vectorSync.enabled

This maintains feature parity between Helm and docker-compose deployments,
allowing users to tune chunking for their embedding models and use cases.

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

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

charts/nextcloud-mcp-server/values.yaml

@@ -314,6 +314,20 @@ vectorSync:
   # Maximum queue size for documents pending indexing
   queueMaxSize: 10000
 
+# Document Chunking Configuration
+# Controls how documents are split into chunks before embedding
+# Only relevant when vectorSync.enabled is true
+documentChunking:
+  # Number of words per chunk (default: 512)
+  # Smaller chunks (256-384): Better for precise searches, more chunks to store
+  # Medium chunks (512-768): Balanced approach (recommended for most use cases)
+  # Larger chunks (1024+): Better for context, less precise matching
+  chunkSize: 512
+  # Number of overlapping words between chunks (default: 50)
+  # Recommended: 10-20% of chunkSize for context preservation across boundaries
+  # Must be less than chunkSize
+  chunkOverlap: 50
+
 # Qdrant Vector Database Configuration
 # Three deployment modes available:
 # 1. Local In-Memory: Fast, ephemeral, zero-config (mode: "memory")