docs: update README for v1.2.0

[kienbui1995]

Update README with v1.2 features: plugin system docs, new commands, per-turn cost.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 6aa19825-996e-42d8-aef9-0f04ef0084ae

📥 Commits

Reviewing files that changed from the base of the PR and between a0af358 and 68a28ed.

📒 Files selected for processing (1)

• README.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/v1.2-readme

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