[Feature]: Support customizable frontmatter key gathering (e.g., categories) and AI-based reuse of existing values #18
Description
Problem Statement
Currently, the project only processes the tags key in frontmatter via src/main.ts#processFrontmatterItem. This means that other common frontmatter keys, such as category or categories, are ignored and cannot leverage the same features or behaviors as tags. This limitation restricts the flexibility for users who would like to uniformly collect and analyze metadata keys beyond just tags.
Proposed Solution
Support gathering customized frontmatter keys (like category or categories) with the same behavior as the existing tags key handling. Enable users to specify which frontmatter keys should be collected and processed in the same way as tags. The set of frontmatter keys to gather should be made configurable in the plugin settings UI. Additionally, scope future AI features to allow reusing existing collected values for specific keys, promoting consistency and intelligence when handling custom metadata fields.
Additional Context
Background: I want to collect values from category/categories in addition to tags in order to provide a richer metadata extraction. Looking ahead, it will be helpful for AI-based features to utilize previously existing values as reference options for custom keys, supporting both reusability and consistency.