Enable global vs. study-level tag distinction in Discovery

Research teams need to organize highlights with both granular, study-specific tags (for individual researcher workflows) and organization-wide tags (for cross-study strategic analysis). Currently, all tags exist in a single namespace, making it difficult to maintain clean taxonomies and filter meaningfully across different organizational levels.

Without tag-level separation, researchers either pollute the global tag list with context-specific tags, or they avoid tagging altogether to prevent clutter. This limits the ability to track strategic themes (like "onboarding issues" or "competitive disadvantage") across multiple studies while still allowing researchers to organize their individual work effectively.

Add the ability to designate tags as either "study-level" (visible/filterable only within that study) or "global" (searchable across all studies in the workspace). When filtering highlights, users could toggle between viewing study-specific tags and global tags, enabling both granular organization and strategic cross-study analysis.