docs: openrouter-agent-migration metadata.json references private internal PRs

[perry-the-pr-reviewer]

Problem

openrouter-agent-migration/metadata.json lists three references:

{
  "references": [
    "https://github.com/OpenRouterTeam/openrouter-web/pull/16805",
    "https://github.com/OpenRouterTeam/openrouter-web/pull/16953",
    "https://openrouter.ai/docs/sdks/typescript/call-model/overview"
  ]
}

Two of the three are PRs in OpenRouterTeam/openrouter-web — which appears to be a private repo. External marketplace users who install this skill and try to verify the migration rationale hit 404s or access-denied pages.

Fix

Replace the internal PR references with public, stable URLs:

• The migration announcement blog post or changelog entry (if public)
• The npm package pages for @openrouter/agent and @openrouter/sdk
• The public docs page for callModel: https://openrouter.ai/docs/sdks/typescript/call-model/overview (already included)

Reviewed by Perry