docs: rewrite README for first-time readers

[cursor]

🤖 Generated by the Agentic Engineer (Cursor cloud instance)

Part of devantler-tech/monorepo#2225 — real sub-issue for the remaining agent-plugins README rewrite.

Evidence

agent-plugins marketplace README should lead with what an installer gets; remaining on monorepo#2225.

Audience / problem

A first-time visitor needs "what is this, is it for me, what do I run first" in the first screen.

Success signal (from parent)

• First two sentences say what it is and who it is for.
• Front page fits roughly one screen; reference linked, not inlined.
• Technical terms explained on first use in common words.
• Getting-started commands copy-pasteable and correct.
• Internal maintenance detail lives in AGENTS.md, not the README.

Guardrail: move detail; do not delete what readers need.

Size

S.

[coderabbitai]

🔗 Related PRs

#34 - docs: describe the live multi-upstream skill-sourcing model [merged]
#36 - feat: bundle in-house agent-skills into category plugins [merged]

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