Add support for multiple namespaces

[blink-so]

This PR addresses issue #5 by adding support for watching pods across multiple namespaces.

Changes

Core Functionality

• Multi-namespace support: CODER_NAMESPACE now accepts comma-separated list of namespaces
• All-namespace support: Empty CODER_NAMESPACE watches all namespaces cluster-wide
• Backward compatibility: Single namespace deployments continue to work unchanged

RBAC Improvements

• Smart RBAC: Automatically uses ClusterRole/ClusterRoleBinding for multi-namespace scenarios
• Namespace-scoped: Uses Role/RoleBinding for single namespace deployments
• Configurable: rbac.clusterWide option for manual override

Helm Chart Updates

• New values: namespaces parameter replaces namespace (backward compatible)
• Automatic detection: Chart determines RBAC scope based on namespace configuration
• Documentation: Comprehensive examples for all use cases

Documentation

• Updated README: Clear examples for single, multiple, and all-namespace deployments
• RBAC guide: Explains permission requirements for different scenarios
• Migration guide: Smooth transition from single to multi-namespace setups

Usage Examples

Single namespace (existing behavior):

helm install coder-logstream-kube coder-logstream-kube/coder-logstream-kube \
    --set url=https://coder.example.com \
    --set namespaces="coder-workspaces"

Multiple namespaces:

helm install coder-logstream-kube coder-logstream-kube/coder-logstream-kube \
    --set url=https://coder.example.com \
    --set namespaces="user1,user2,user3"

All namespaces:

helm install coder-logstream-kube coder-logstream-kube/coder-logstream-kube \
    --set url=https://coder.example.com \
    --set namespaces=""

Testing

• Code compiles successfully
• Maintains backward compatibility
• Added unit tests for namespace parsing logic

Fixes #5