feat: add cbt plugin for ClickHouse Build Tool access

[reldothescribe]

This PR adds a CBT (ClickHouse Build Tool) plugin to the MCP server.

Features

• List CBT instances
• List models with optional filtering by type and database
• Get model details including dependencies, interval config, and schedules
• Query transformation status (intervals processed, gaps, etc.)
• Get deep links to CBT UI for models

Files Added/Modified

• plugins/cbt/config.go - Configuration for CBT instances
• plugins/cbt/plugin.go - Main plugin implementation
• plugins/cbt/examples.go - Examples loader
• plugins/cbt/examples.yaml - Example queries
• plugins/cbt/python/cbt.py - Python module for sandbox
• pkg/proxy/handlers/cbt.go - Proxy handler for CBT API
• pkg/proxy/server.go - Added CBT routes and datasource methods
• pkg/proxy/server_config.go - Added CBT configuration
• pkg/server/builder.go - Registered CBT plugin
• sandbox/Dockerfile - Added CBT Python module
• sandbox/ethpandaops/ethpandaops/init.py - Added CBT module support

Usage

from ethpandaops import cbt
models = cbt.list_models()
model = cbt.get_model('analytics', 'block_propagation')
status = cbt.get_transformation_status('analytics', 'block_propagation')
url = cbt.get_model_ui_url('analytics', 'block_propagation')