Scanner: Add plugin configuration system with per-plugin settings

[Kami-no-san]

Description

Enable per-plugin configuration (e.g., severity overrides, enabled/disabled, custom thresholds) via the PluginRegistry.

Files to Modify

• packages/scanner-core/src/plugin-registry.ts — add config support
• packages/scanner-core/src/scanner.ts — pass config to plugins
• packages/scanner-types/src/index.ts — extend IRulePlugin or add config types

Acceptance Criteria

• PluginRegistry.register() accepts an optional config object
• Plugin config can override severity of findings
• Plugin config can disable specific detection patterns within a plugin
• Config is validated against a schema defined in plugin metadata
• getPluginConfig(pluginId) method retrieves current config
• Unit tests cover config registration, retrieval, and overrides
• Backward compatible — existing plugins work without config