Refine Mission Control messaging and terminology

[self9dmin]

What does this PR do?

Updates the README to clarify and simplify the Mission Control value proposition and core messaging. Changes include:

• Removes "natively" from platform description for accuracy
• Replaces "Playground" reference with more direct language about the real platform
• Simplifies mission descriptions (e.g., "navigating the actual Dynatrace UI" → "navigating the actual UI")
• Changes "earn XP, unlock achievements" to "earn XP, clear checkpoints" for consistency with current terminology
• Refines the core belief statement from "designed around a single belief" to "The idea is simple" for better readability
• Updates task language from "completing real tasks" to "doing real work" for clarity

Type of change

• Bug fix
• New mission
• UI improvement
• Infrastructure / tooling

Checklist

• TypeScript compiles clean (npx tsc --noEmit)
• Tested locally with npx dt-app dev
• No changes to app.config.json
• Branch follows naming convention (feat/, fix/, chore/)

Test Plan

N/A - Documentation update only

https://claude.ai/code/session_019n51rzmZLQAUpJZ3dfwVVU