fix: make /understand discoverable in Claude Code VS Code extension

[jasoncobra3]

Summary

Fixes #505

The /understand command was available when installed through the Claude Code CLI but was not discoverable in the Claude Code VS Code extension.

Root Cause

The Claude plugin manifests were missing explicit skills and agents entries.

While the CLI can discover skills through directory scanning, the VS Code extension relies on manifest-based discovery. Without these entries, the extension could not locate and register the /understand skill.

Changes

• Added skills and agents entries to .claude-plugin/plugin.json
• Added skills and agents entries to understand-anything-plugin/.claude-plugin/plugin.json

Verification

• Rebuilt @understand-anything/core
• Rebuilt @understand-anything/skill
• Verified successful builds
• Confirmed the manifest paths correctly expose skill and agent directories for extension discovery

This change is backward compatible and does not alter existing CLI behavior.

[jasoncobra3]

Opened PR #511 with a proposed fix.

Root cause was that the Claude Code plugin manifests were missing the explicit skills and agents entries used for extension-based discovery.

The CLI could still discover the skill through directory scanning, but the VS Code extension could not register /understand without the manifest entries.