chore(ci): prune upstream workflows and add upstream sync for fork

[terisuke]

Problem

Our fork carries 27 upstream-only GitHub Actions workflows (deploy, publish, beta, nix, containers, storybook, discord, vouch, triage, etc.) that never run successfully in our fork context and clutter the Actions tab.

Proposal

1. Remove the 27 unused upstream workflows
2. Keep the 5 workflows we actually use: typecheck, test, pr-standards, pr-management, generate
3. Simplify test.yml to Linux-only (remove Windows matrix)
4. Add an upstream-sync.yml workflow that weekly fetches from anomalyco/opencode while excluding .github/workflows/ to preserve our fork's custom CI