Remove/disable the plugin-config command (stream-route stays)

[shreemaan-abhishek]

Part of #22.

plugin-config is NOT exposed by the API7 EE control-plane management API (verified). Remove or disable it the same way PR #21 handled upstream/consumer-group/service-template:

• pkg/cmd/plugin-config command + registration in pkg/cmd/root/root.go
• config dump/diff/sync handling
• declarative-config validation: reject top-level plugin_configs
• docs/user-guide/plugin-config.md and any skill references
• E2E coverage asserting the command is not exposed

Note: the CP still accepts plugin_configs inside batch config-validation payloads — do not reject it there. stream-route stays as-is (it is exposed).

[coderabbitai]

🔗 Related PRs

#21 - Remove unsupported API7 resource commands [closed]

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