[Feature]: Clarify installation steps for Better Harness plugin in Qoder CLI

[iyongliang]

Preflight

• I searched existing issues for a similar request.

What problem are you trying to solve?

Description

The current README is somewhat vague regarding plugin installation and does not clearly distinguish between the Desktop and pure CLI environments. Developers using only qodercli often struggle to find the correct installation source (e.g., mistakenly searching the npm registry).
It would be highly beneficial to add explicit installation instructions for the pure CLI environment to the README.
Suggested Documentation Update
I recommend adding the following steps under the Installation or Quick Start section for pure CLI users:

Pure Qoder CLI Installation Steps:

From qodercli command

1. Add the plugin marketplace source:

qodercli plugin marketplace add 'https://github.com/QoderAI/better-harness.git'

2. Install the Better Harness plugin:

qodercli plugin install better-harness@better-harness

3. Verify the installation:

qodercli plugin list

From git

mkdir -p ~/.qoder/plugins/marketplaces/
cd ~/.qoder/plugins/marketplaces/
git clone https://github.com/QoderAI/better-harness.git
qodercli plugin install ~/.qoder/plugins/marketplaces/better-harness
Installing plugin "/home/YOUR_NAME/.qoder/plugins/marketplaces/better-harness"...
Plugin "better-harness@local" installed successfully.

What would success look like?

Plugin "better-harness@local" installed successfully.
/better-harness worked in qodercli, without qoder desktop

Where would this be used?

Some server or remote machine, cannot install qoder desktop software.

Examples or evidence

No response

Anything else?

README not clarify

README 写的不太清楚

[phodal]

@iyongliang welcome to PR