Migrate from OpenResponses to Chat Completions API

[xamdel]

Summary

• Replace OpenResponses flat-item format with standard Chat Completions message-based format
• Rewrite inference-handler.ts from 725 → 298 lines by eliminating bidirectional format conversion (no longer needed since plugin calls pi-ai directly)
• Update consumer client, proxy, protocol schemas, and tests to match

What changed

File	Change
schemas/chatcompletions.ts	New schema: MessageSchema, ChatCompletionRequestSchema, ChatCompletionSchema, ChatCompletionChunkSchema
schemas/openresponses.ts	Deleted (343 lines)
inference-handler.ts	buildContext takes messages[] directly; streaming emits chat.completion.chunk; non-streaming returns choices[].message
sixerr-client.ts + types.ts	messages instead of input; /v1/chat/completions path; max_tokens field
protocol.ts	Usage fields: prompt_tokens/completion_tokens
http-proxy.ts	Route path → /v1/chat/completions
schemas.test.ts	25 tests for new schema (tools, tool results, marketplace extensions, old format rejection)

Server-side migration guide: sixerr-server/CHAT-COMPLETIONS-MIGRATION.md

Test plan

• pnpm tsc --noEmit — clean
• pnpm test — 41/41 pass
• Review: non-streaming response has choices[0].message with optional tool_calls
• Review: streaming emits chat.completion.chunk with delta field
• Review: scanner on server side can still parse the response

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