feat: add antigravity workflows playbooks and orchestration skill

Introduce the first Antigravity Workflows foundation with machine-readable workflow metadata, a dedicated orchestration skill, and onboarding docs that explain when to use bundles versus workflows. This reduces multi-skill friction for common goals like SaaS MVP delivery, security audits, AI agent builds, and browser QA.

Co-authored-by: Cursor <cursoragent@cursor.com>

skills/antigravity-workflows/resources/implementation-playbook.md

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+# Antigravity Workflows Implementation Playbook
+
+This document explains how an agent should execute workflow-based orchestration.
+
+## Execution Contract
+
+For every workflow:
+
+1. Confirm objective and scope.
+2. Select the best-matching workflow.
+3. Execute workflow steps in order.
+4. Produce one concrete artifact per step.
+5. Validate before continuing.
+
+## Step Artifact Examples
+
+- Plan step -> scope document or milestone checklist.
+- Build step -> code changes and implementation notes.
+- Test step -> test results and failure triage.
+- Release step -> rollout checklist and risk log.
+
+## Safety Guardrails
+
+- Never run destructive actions without explicit user approval.
+- If a required skill is missing, state the gap and fallback to closest available skill.
+- When security testing is involved, ensure authorization is explicit.
+
+## Suggested Completion Format
+
+At workflow completion, return:
+
+1. Completed steps
+2. Artifacts produced
+3. Validation evidence
+4. Open risks
+5. Suggested next action