[FEATURE] Add a tag explorer to the VS Code sidebar

[mors119]

Summary

Add a tag-focused view to the FrilVault VS Code sidebar.

The view should group notes by tag and display the number of notes associated with each tag.

Example

TAGS

architecture (12)
performance   (5)
todo          (18)
parser        (9)

Expanding a tag:

todo (18)

├─ parser.rs
│  └─ Improve error recovery
├─ main.rs
│  └─ Replace temporary initialization
└─ config.rs
   └─ Validate environment values

Requirements

• List all tags used by the current workspace.
• Display note counts per tag.
• Sort tags predictably, initially alphabetically.
• Expand a tag to show its notes.
• Show file path, anchor information, and a short content preview.
• Open and reveal the note anchor when a note is selected.
• Refresh after note creation, editing, deletion, or vault reload.
• Reuse tag aggregation logic from frilvault-core.

Empty State

When no tagged notes exist, show a useful message explaining how to add tags.

Acceptance Criteria

• The sidebar displays all tags in the workspace.
• Each tag displays the correct note count.
• Expanding a tag displays matching notes.
• Selecting a note navigates to its anchor.
• The view refreshes after tag changes.
• Duplicate tag entries are not shown.
• Line and symbol anchors are supported.

Design Principle

Tags should provide an alternate knowledge navigation path independent of the file tree.