feat(discord): add merge notification workflow

[egdev6]

Summary

• Adds a GitHub Actions workflow that posts a Discord embed when changes land on main.
• Summarizes the associated PR/commit, author, component scope, docs/tests/CI tags, and Storybook link.
• Skips safely when DISCORD_WEBHOOK_URL is not configured.

Closes #245

Review scope

• Primary files/areas:
  • .github/workflows/discord-merge-notify.yml
• Out of scope:
  • Discord server setup and the actual webhook secret value.
  • Component/runtime code changes.
• Review workload: small — one new workflow file.

Type of change

• New component
• Existing component update/refactor
• Bug fix
• Accessibility
• Design tokens
• Storybook/docs
• Infrastructure/tooling
• NPM/dependencies

Workflow gates

• Linked issue is present with Closes #NNN or maintainer approved an exception.
• Linked issue has label status:approved before implementation starts.
• Work was started through the Project flow: assignee set, Project status In progress, branch/worktree recorded.
• PR title follows Conventional Commit format: <type>(<optional scope>): <description>.
• Commits follow the same commitlint-enforced format.
• Diff is focused; unrelated work is called out in Review scope.
• MCP/runtime artifacts are absent: .playwright-mcp, page-*.png, page-*.jpeg, *.md.playwright-output.

Component evidence (required for component changes)

N/A — infrastructure-only workflow change.

Accessibility evidence

N/A — no UI/runtime component behavior changed.

Validation evidence

• TypeScript: pnpm exec tsc --noEmit passed via lefthook pre-commit.
• Unit tests: pnpm test passed via lefthook pre-push — 28 files, 440 tests.
• Build/package: N/A — workflow-only change.
• Storybook or visual check: N/A — no Storybook/component changes.
• Accessibility check: N/A — no UI changes.
• Security/dependency check:
  • Workflow reads DISCORD_WEBHOOK_URL from GitHub Actions secrets only.
  • The secret is never printed or included in the Discord payload.
  • Workflow skips safely when the secret is absent.
• Workflow syntax:
  • Python YAML parse passed locally.
  • git diff --check passed.
  • Fresh reviewer extracted the github-script body and node --check passed.
  • actionlint was unavailable locally.

Screenshots or recordings

N/A — Discord webhook requires repository secret configuration and runs on push to main.

Notes for reviewer

To enable notifications after merge, add repository secret:

DISCORD_WEBHOOK_URL=<Discord channel webhook URL>