feat(discord): improve merge summary message

[egdev6]

Summary

• Adds a clear Spanish Qué cambió field to Discord merge notifications.
• Extracts useful bullets from the PR ## Summary section when available.
• Falls back to pushed commit messages and includes a short changed-files preview for direct pushes.

Closes #249

Review scope

• Primary files/areas:
  • .github/workflows/discord-merge-notify.yml
• Out of scope:
  • Discord secret setup.
  • Component/runtime changes.
• Review workload: small — one 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.
• Storybook or visual check: N/A.
• Accessibility check: N/A.
• Security/dependency check: no dependency changes; webhook secret remains read from DISCORD_WEBHOOK_URL only.
• Workflow syntax:
  • Python YAML parse passed locally.
  • Extracted github-script body passed node --check.
  • git diff --check passed.
• Review: fresh-context reviewer found no blockers; applied its direct-push multi-commit file summary suggestion before commit.

Screenshots or recordings

N/A — message will be visible on the next push/merge to main after this PR merges.

Notes for reviewer

The Discord embed now includes:

• Qué cambió — PR summary bullets or commit-message fallback.
• Archivos — changed-file count plus up to 8 file paths.
• Existing author, commit, scope, and Storybook fields.