feat: add pluggable multi-CLI backend system

Implement BackendAdapter interface with four CLI backends:
- ClaudeCodeBackend (extracted from AgentRuntime)
- CodexBackend (OpenAI Codex CLI)
- GeminiBackend (Google Gemini CLI)
- OpenCodeBackend (OpenCode CLI)

Add BackendRegistry for resolution/creation via AGENT_BACKEND env var.
Refactor AgentRuntime to delegate to BackendAdapter instead of
hardcoding Claude CLI. Update GatewayConfig with new env vars
(AGENT_BACKEND, BACKEND_CLI_PATH, BACKEND_MODEL, BACKEND_MAX_TURNS).

Includes 10 property-based test files and unit tests for edge cases.

.env.example

@@ -5,7 +5,11 @@ DISCORD_BOT_TOKEN=your-discord-bot-token-here
 OUTPUT_CHANNEL_ID=your-discord-channel-id-for-heartbeat-cron-output
 
 # Optional
-# CLAUDE_CLI_PATH=claude
+# CLAUDE_CLI_PATH=claude          # Deprecated: use BACKEND_CLI_PATH instead
+# AGENT_BACKEND=claude            # claude | codex | gemini | opencode
+# BACKEND_CLI_PATH=claude         # Path to CLI binary (defaults to backend name)
+# BACKEND_MODEL=                  # Optional model override for the active backend
+# BACKEND_MAX_TURNS=25            # Max agentic turns per query
 # CONFIG_DIR=./config
 # ALLOWED_TOOLS=Read,Write,Edit,Glob,Grep,WebSearch,WebFetch
 # PERMISSION_MODE=bypassPermissions