## Description
This PR adds support for selectively disabling tools via the DISABLED_TOOLS environment variable, allowing users to customize which MCP tools are available in their Zen server instance. This feature enables better control over tool availability for security, performance, or organizational requirements.
## Changes Made
- [x] Added `DISABLED_TOOLS` environment variable support to selectively disable tools
- [x] Implemented tool filtering logic with protection for essential tools (version, listmodels)
- [x] Added comprehensive validation with warnings for unknown tools and attempts to disable essential tools
- [x] Updated `.env.example` with DISABLED_TOOLS documentation and examples
- [x] Added comprehensive test suite (16 tests) covering all edge cases
- [x] No breaking changes - feature is opt-in with default behavior unchanged
## Configuration
Add to `.env` file:
```bash
# Optional: Tool Selection
# Comma-separated list of tools to disable. If not set, all tools are enabled.
# Essential tools (version, listmodels) cannot be disabled.
# Available tools: chat, thinkdeep, planner, consensus, codereview, precommit,
# debug, docgen, analyze, refactor, tracer, testgen
# Examples:
# DISABLED_TOOLS= # All tools enabled (default)
# DISABLED_TOOLS=debug,tracer # Disable debug and tracer tools
# DISABLED_TOOLS=planner,consensus # Disable planning tools
