feat(gateway): add OpenResponses /v1/responses endpoint

Add a new `/v1/responses` endpoint implementing the OpenResponses API
standard for agentic workflows. This provides:

- Item-based input (messages, function_call_output, reasoning)
- Semantic streaming events (response.created, response.output_text.delta,
  response.completed, etc.)
- Full SSE event support with both event: and data: lines
- Configuration via gateway.http.endpoints.responses.enabled

The endpoint is disabled by default and can be enabled independently
from the existing Chat Completions endpoint.

Phase 1 implementation supports:
- String or ItemParam[] input
- system/developer/user/assistant message roles
- function_call_output items
- instructions parameter
- Agent routing via headers or model parameter
- Session key management

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -35,6 +35,7 @@ Docs: https://docs.clawd.bot
 - **BREAKING:** Reject invalid/unknown config entries and refuse to start the gateway for safety; run `clawdbot doctor --fix` to repair. 
 
 ### Changes
+- Gateway: add `/v1/responses` endpoint (OpenResponses API) for agentic workflows with item-based input and semantic streaming events. Enable via `gateway.http.endpoints.responses.enabled: true`.
 - Usage: add `/usage cost` summaries and macOS menu cost submenu with daily charting.
 - Agents: clarify node_modules read-only guidance in agent instructions.
 - TUI: add syntax highlighting for code blocks. (#1200) — thanks @vignesh07.