fix: restore the plugin CLI analytics labels and drop the unused README files

[ziyiunity]

What

The automated skills sync in #56 mirrors the skills repo directory wholesale, so it reverted two plugin-only changes:

• The --caller plugin --skill <name> labels added in feat: label CLI invocations with caller and skill (blocked on the next CLI release) #31 were removed from all 10 unity command examples across the 8 CLI-driving skills, along with the unity-cli paragraph telling agents to pass them on every invocation. Without them, CLI usage driven by the plugin can't be attributed in analytics.
• It re-added implement-in-app-purchases/README.md, levelplay-unity-integration/README.md and levelplay-unity-integration/CHANGELOG.md. None of them is referenced by a SKILL.md, so agents never read them; they were deliberately left out of the plugin.

Change

• Reapplies the 10 labels and the unity-cli instruction paragraph exactly as in feat: label CLI invocations with caller and skill (blocked on the next CLI release) #31. Each label lands on the same example line feat: label CLI invocations with caller and skill (blocked on the next CLI release) #31 labeled; the rest of sync: align skills catalog with Unity-Technologies/skills #56's content (the CLI beta.11 skill refresh) is kept.
• Removes the three unused files.

Verify

git grep -- '--caller plugin --skill' -- skills

Expect 10 labeled examples in 8 skills plus the unity-cli paragraph.