fix: auto-turn streaming, plan sanitization, and HTTP response close in proxy

Fix: auto-turn with server-side tool execution
- Rewrite _proxyStreaming to auto-turn internally: forward reasoning/text
  immediately, execute tools server-side (Glob/Read/Grep/Write/Edit/Bash/Task),
  loop until final text-only response

Fix: shouldContinue matching API finishReason format
- API sends 'tool-calls' (hyphen), not 'tool_calls' (underscore)
- Match both variants in shouldContinue check and _mapFinishReason

Fix: duplicate tool call counting
- tool-call event is redundant with tool-input-end; skip it to prevent
  duplicate tool_call_id errors in subsequent API rounds

Fix: HTTP response never closed after [DONE]
- Add await response.close() after writing [DONE] so OpenCode client
  does not hang on 'loading' forever

Fix: plan artifact sanitization
- Remove plan header, content, fragment, and trailer lines from chunks
- Apply stop-token patterns ([endchat], [done], etc.)

Author: Khip01