[FEATURE] Add tag rename, merge, and cleanup operations

[mors119]

Summary

Add workspace-wide tag management operations.

As a project evolves, tags may be renamed, duplicated, misspelled, or replaced by a better classification. Users need a safe way to maintain tag consistency across all notes.

Operations

Rename

performance → optimization

Replace the tag across all matching notes.

Merge

bug + defect → issue

Move notes from one or more source tags into a target tag and remove duplicates.

Remove

Remove a tag from every note after explicit confirmation.

Find unused tags

If FrilVault later stores a tag registry or color metadata, identify tags no longer attached to any note.

Requirements

• Implement transformations in frilvault-core.
• Update notes atomically where practical.
• Avoid duplicate tags after rename or merge.
• Report the number of affected notes.
• Provide a preview or confirmation before destructive changes.
• Preserve unrelated note fields and formatting.
• Refresh VS Code views and indexes after completion.
• Support CLI commands or an equivalent command structure.

Possible CLI

flvt tag rename todo task
flvt tag merge bug defect --into issue
flvt tag remove legacy
flvt tag list --unused

Exact command naming may follow the existing CLI conventions.

Acceptance Criteria

• Tags can be renamed across the workspace.
• Multiple tags can be merged safely.
• Tags can be removed after confirmation.
• Duplicate tags are eliminated.
• The number of affected notes is reported.
• Failed writes do not silently leave partially updated data.
• Tests cover conflicts, duplicates, and empty matches.

Design Principle

Tag organization must remain maintainable as the vault grows.