Add interactive front-end shell menu to notes_app CLI

[Copilot]

The notes_app CLI lacked an interactive REPL-style interface — users had to invoke each command as a separate subprocess call. This adds a persistent interactive shell exposing all available commands in a single session.

Changes

python/src/notes_app/cli/shell.py (new)

• Interactive notes> prompt loop backed by the full NoteService / BackupService stack
• Dispatches all commands: help, list, create, search, read, update, delete, backup, restore, quit
• Uses shlex.split() for correct quoted-argument parsing
• Handles EOF (Ctrl+D) and KeyboardInterrupt (Ctrl+C) gracefully
• Entrypoint: python -m notes_app.cli.shell

python/tests/test_shell.py (new)

• 23 unit tests covering every command, missing-argument error paths, unknown commands, empty input, unmatched quotes, and interrupt handling

Example

$ python -m notes_app.cli.shell
Future Proof Notes Manager - Interactive Shell
Type 'help' for available commands, 'quit' to exit.

notes> create "Meeting Notes" "Discussed Q3 roadmap"
Created note 'meeting-notes.md'
notes> list
Notes:
============================================================
meeting-notes.md
  Title: Meeting Notes
  Modified: 2026-04-30T15:07:43Z
1 note(s) found.
notes> update meeting-notes --tags "q3,roadmap"
Updated note 'meeting-notes.md'
notes> quit

Goodbye!