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.

.kiro/specs/multi-cli-backend/.config.kiro

@@ -0,0 +1 @@
+{"specId": "66d67457-3ea3-493c-9d0f-b868b51d309d", "workflowType": "requirements-first", "specType": "feature"}