Remove subcommands, make musup TUI-only with auto-scan

[Jason-Abbott]

• Remove cmd/scan.go and scan subcommand registration
• Remove runCheck / CLI check mode from cmd/root.go
• Simplify cmd/root.go to TUI-only with flag package, pass scan func to TUI
• Add ScanFunc type and scan state to TUI model
• Start scan on Init(), reload artists every 500ms during scan
• Show scanning status in TUI (spinner + SCANNING in help bar, empty state message)
• Handle scan completion, update empty-state message
• Tests pass
• Build, test, lint clean
• Update README and CLAUDE.md

Summary of Changes

Removed all subcommands (scan, check/years, version via Cobra). Replaced Cobra with stdlib flag. musup now runs only as a TUI. On launch it auto-scans the CWD recursively, populating the view in real-time (re-queries DB every 500ms during scan). Added - key for VACUUM. Updated README and CLAUDE.md.

👾 This issue is managed by Jig. Edits made here will be overwritten.