Add OpenAI Agents API support

[mkht]

Summary

• add 30 commands for the OpenAI Agents API public beta
• support reusable agents, environment templates and live environment files
• support managed sessions, input events, SSE event streams, items, turns, subagents, and artifacts
• support agent vaults and credential lifecycle operations
• send the required OpenAI-Beta: agents=v1 header on every Agents API request
• support cursor pagination, opaque environment-file page tokens, binary artifact downloads, and OpenAI-only validation
• update manifest exports, README, command documentation, and generated external help

The September 2026 P0 compatibility changes are intentionally handled in a separate PR.

Validation

• Agents Offline tests: 11 passed, 0 failed
• PSScriptAnalyzer with PSScriptAnalyzerRules.psd1: 0 findings
• all 30 commands load from the module and have Markdown help
• README links and cached diff check passed