fix: normalize tool schemas so required ⊇ properties for OpenAI/Codex

[gnanam1990]

Problem

OpenAI and Codex enforce strict JSON Schema validation — every key in properties must also appear in required. Anthropic schemas often mark fields as optional (omitted from required), which causes 400 errors immediately on any OpenAI/Codex endpoint.

Reported in #46. Example error:

API Error: Codex API error 400: {
  "error": {
    "message": "Invalid schema for function 'Agent': In context=(), 'required' is required to be supplied and to be an array including every key in properties. Missing 'subagent_type'.",
    "type": "invalid_request_error",
    "param": "tools[0].parameters",
    "code": "invalid_function_parameters"
  }
}

The Agent tool has subagent_type in properties but not in required — valid for Anthropic, rejected by OpenAI.

Fix

Added normalizeSchemaForOpenAI() in convertTools() in openaiShim.ts. Before sending any tool schema to the API, it ensures required is a superset of all properties keys:

• Existing required entries are preserved
• Missing keys are appended
• Schemas without properties (e.g. type: 'string') pass through unchanged

This fixes the issue for the Agent tool and any other tool with optional properties, without touching individual tool definitions.

Test

Tested locally with CLAUDE_CODE_USE_OPENAI=1 and GPT-4o:

spawn an agent to list files in current directory

Before fix: immediate 400 error on tool schema validation
After fix: Agent(List files in current directory) → Done (1 tool use · 8.9k tokens · 8s)

Closes #46.