feat: align terminal AI with OpenRouter config

[MishukAdhikari]

Craft Mishuk — align xCloud Terminal with issue #5099 AI direction

Requested by Master Mishuk Adhikari: check whether xCloudDev/xcloud-terminal is aligned with xCloud issue #5099 / the OpenRouter-backed AI branch, and bring the terminal-side entrypoint closer to that direction.

Summary

• Replaced the old AI scaffold response with a real terminal-side AI entrypoint for one-shot prompts.
• Added OpenRouter/OpenAI-compatible and Anthropic provider config loading.
• Supports the xCloud AI branch env naming:
  • OPENROUTER_API_KEY
  • OPENROUTER_API_URL=https://openrouter.ai/api/v1
  • OPENROUTER_DEFAULT_MODEL
• Keeps portable aliases:
  • XCLOUD_AI_PROVIDER
  • XCLOUD_AI_API_KEY
  • XCLOUD_AI_BASE_URL
  • XCLOUD_AI_MODEL
• Added xterm doctor to report provider/model/base URL/configured status with redacted keys only.
• Anchored the AI system prompt to issue #5099: self-support, read-only diagnostics first, safe CLI/Public API capability layer, confirmations for destructive/billing-impacting actions, and no secrets in prompts/logs.
• Updated README and architecture docs for the current alignment and env setup.
• Added smoke coverage for AI setup guidance, OpenRouter config, doctor redaction, and hosted skills repo display.

Alignment note

This PR does not merge the big Laravel AI/chatbox branch into the standalone terminal repo. That branch belongs in xCloudDev/xCloud for the in-app chatbox/gateway. This PR makes xcloud-terminal the terminal-side entrypoint that is compatible with the same provider/env and safety direction.

Verification

• git diff --check
• node --check bin/xterm.js
• node --check lib/ai/config.js
• node --check lib/ai/provider.js
• npm test
• npm pack --dry-run
• node ./bin/xterm.js --ai
• node ./bin/xterm.js doctor
• node ./bin/xterm.js --ai why is my site down