refactor: remove plugin.json manifest support

[christso]

Summary

plugin.json (the plugin manifest with name, version, description, author, license) is largely redundant in allagents. The only functional use is in getPluginName() which reads the name field, falling back to the directory name.

Since allagents uses workspace.yaml for all plugin configuration (source, clients, install mode, exclude patterns), plugin.json is an unnecessary Claude-specific artifact.

Proposed Changes

• Simplify getPluginName() to always return basename(pluginPath) (directory name)
• Remove PluginManifestSchema and PluginManifest type from src/models/plugin-config.ts
• Remove plugin.json reads from src/core/plugin.ts
• Update tests that create plugin.json fixtures (~10 test files)

Context

Identified during #178 / PR #179 — exclude patterns were initially added to plugin.json before being moved to workspace.yaml where they belong.