Add ImageJAI plugin page

[Jay2owe]

Summary

Add an ImageJAI plugin page describing the embedded assistant and standalone console, cloud/local model selection, automatic Fiji startup, session macros, privacy controls and direct TCP scripting.

The ImageJ-AI update site is live and a fresh official Fiji installation has installed and tested its 0.5.0 JAR.

Validation

• Checked the menu and settings labels and console controls against the plugin source and user guide.
• Parsed the YAML front matter and checked the version, release date, licence, source and update-site fields.
• Checked the page for personal paths and unsupported claims.
• git diff --check passed.
• This pull request changes only _pages/plugins/imagejai.md.