README: optimize for search discovery

[George-iam]

Summary

• Expand intro with high-value search terms (workflow orchestration, human approval gates, crash recovery, Temporal alternative)
• Rename "The Problem" to "Why AXME?" with descriptive bullet headers for better scanability
• Add Inngest to comparison table for broader competitive context
• Add GitHub Topics section with 15 discovery keywords
• Rename dashboard section to include "Agent Monitoring Dashboard" for search targeting
• All changes use natural language - no keyword stuffing

Test plan

• Verify README renders correctly on GitHub
• Check all links resolve (mesh.axme.ai, cloud.axme.ai, etc.)
• Confirm Topics section displays as inline code blocks