Add plugin/extension system for custom error rules

[naheel0]

Title: Add plugin/extension system for custom error rules

Problem

Contributing patterns requires editing lib/database.json. Teams need a safer, extensible way to add rules.

Proposed change

Support external plugin files or a plugin folder (e.g., .errlens/plugins/*.json) that are loaded at runtime with a simple plugin schema.

Files to add/change

• lib/plugin-loader.js (new) - loads and validates plugin files
• update lib/matcher.js to merge plugin patterns at startup
• add docs/examples in examples/plugins/

Tasks

• Define plugin schema (name, patterns, priority).
• Load plugins from configured paths and merge with DB.
• Add security note in docs: only load local/trusted plugins.

Acceptance criteria

• Dropping a JSON plugin file results in its patterns being considered in analyze results.

Labels: enhancement, community