docs(openclaw-plugin): refresh architecture guides with diagrams

[qin-ctx]

Description

Refresh the OpenClaw plugin architecture guides so they describe the current implementation more clearly, and add diagram assets that explain the main runtime flows.

Related Issue

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update
• Refactoring (no functional changes)
• Performance improvement
• Test update

Changes Made

• Reworked examples/openclaw-plugin/README.md into an implementation-focused architecture guide for the current plugin behavior.
• Reworked examples/openclaw-plugin/README_CN.md to match the same structure and clarify the current runtime, recall, session, and routing model.
• Added four PNG diagrams for overall architecture, prompt-front recall flow, session lifecycle, and local/remote routing behavior.

Testing

• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have tested this on the following platforms:
  • Linux
  • macOS
  • Windows

Manual checks performed:

• Ran git diff --check
• Reviewed the generated PNG diagrams visually after layout adjustments

Checklist

• My code follows the project's coding style
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

Added architecture screenshots directly under examples/openclaw-plugin/images/ and referenced them from both plugin READMEs.

Additional Notes

This PR intentionally keeps only the final PNG assets in the repository and does not include the temporary local generation helper used during drafting.

[github-actions]

Failed to generate code suggestions for PR