Replace Copilot CLI with opencode (Mistral provider)

[jfomhover]

Switches the daily research monitor workflow from Copilot CLI to opencode.

Changes:

• Install opencode (
  pm install -g opencode-ai) instead of Copilot CLI
• Auth via \MISTRAL_API_KEY\ secret instead of \COPILOT_GITHUB_TOKEN\
• Run agent via \opencode run --auto --model mistral/mistral-small-latest\
• Output to separate files (\ itle.txt\ + \�ody.md) instead of fragile JSON-in-string
• Updated prompt preamble and instructions for opencode
• Increased agent timeout to 30min

Tested:

• Local end-to-end test with Mistral Large (feed download → agent → output files)
• GitHub Actions run with Mistral Small completed agent step in ~2min (discussion step fails — \DISCUSSIONS_WRITE_TOKEN\ not configured)