Update Linear agent session with external URLs

Posts external URLs (PR links, preview URLs, etc.) to the Linear agent session for display in the Linear UI. Sessions without a `linearSessionId` (governor-generated sessions) return `{ updated: false }` without error. Fail-closed: Linear API errors return 500.

curl -X POST "https://example.com/api/sessions/string/external-urls" \  -H "Content-Type: application/json" \  -d '{    "externalUrls": [      {        "label": "string",        "url": "http://example.com"      }    ]  }'

{
  "updated": true,
  "externalUrls": [
    {
      "label": "string",
      "url": "string"
    }
  ],
  "reason": "string"
}

{
  "error": "Session not found"
}

{
  "error": "Session not found"
}

{
  "error": "Session not found"
}

{
  "error": "Session not found"
}