[FEATURE] Search notes by tag across CLI and VS Code

[mors119]

Summary

Add tag-based note search to frilvault-core, the CLI, and the VS Code extension.

Tags are currently stored as metadata, but users cannot use them to locate related notes. This issue turns tags into a practical navigation mechanism.

Proposed Behavior

CLI

flvt search --tag todo
flvt search --tag architecture

Each result should include the source file, anchor, content preview, and tags.

VS Code

Add a command such as:

FrilVault: Search Notes by Tag

Selecting a result should open the source file and reveal the attached anchor.

Requirements

• Search notes by one exact tag.
• Treat todo and #todo consistently.
• Reuse shared search logic from frilvault-core.
• Support both line and symbol anchors.
• Show a clear empty state when no notes match.
• Keep CLI and VS Code results consistent.
• Preserve compatibility with existing vault data.

Acceptance Criteria

• flvt search --tag <tag> returns matching notes.
• VS Code provides tag-based note search.
• Selecting a result navigates to the note anchor.
• Tag normalization is tested.
• Core and integration tests cover matching and empty results.

Design Principle

Tags should help users retrieve knowledge, not merely decorate note metadata.