docs(readme): add installation guide for qodercli

[iyongliang]

Summary

Update documents, add installation better-harness guideline when only qodercli

Why

• Issue: [Feature]: Clarify installation steps for Better Harness plugin in Qoder CLI #74

Traceability and Scope

No

Change Type

• Feature
• Bug fix
• Tests only
• Documentation/community
• Refactor with no intended behavior change
• Dependency, packaging, or infrastructure

Test and Review Evidence

Manual or visual evidence:

Risk and Recovery

Remove all old install methods

AI Involvement

Checklist

• I followed AGENTS.md, CONTRIBUTING.md, and the relevant canonical-owner guidance.
• The change is focused and does not include unrelated local or generated state.
• Tests and documentation match the behavior actually delivered.
• Markdown links were checked when documentation moved or changed.
• Cross-platform behavior was considered for Windows, macOS, and Linux.
• Package/runtime verification was run when shipped files or dependencies changed.
• User-facing or compatibility changes are recorded in CHANGELOG.md.
• I have the right to contribute this work under the repository's MIT License.