fix(core): treat empty notes content as no notes in task detail

[Sanjays2402]

Description

GetTaskDetailUseCase computed has_notes from notes_content is not None, but NotesController.delete_notes implements deletion as write_notes(task_id, ""), leaving an empty row behind. SqliteNotesRepository.has_notes and get_task_ids_with_notes both require content != "", so after deleting a note the two sources of truth disagreed: the task list showed no note indicator while show/detail reported has_notes=True with empty content. Fixed by using bool(notes_content).

Related Issue

Closes #1157

Type of Change

• Bug fix (non-breaking change which fixes an issue)

Changes Made

• get_task_detail.py: has_notes = bool(notes_content) so an empty (deleted) note counts as no notes, matching NotesRepository.has_notes().
• Added test_execute_after_notes_deleted regression test.

Testing

Test Environment

• OS: macOS
• Python version: 3.14
• UV version: N/A (pytest run directly against the source tree)

Tests Performed

• Unit tests pass (make test)
• Added new tests for new features
• Manual testing completed
• All affected commands work as expected

The new test fails without the source change (has_notes=True with notes_content='') and passes with it, verified by stashing the source file. Full taskdog-core suite: 1166 passed.

Code Quality Checklist

• Linter passes (make lint)
• Type checker passes (make typecheck)
• Code is formatted (make format)
• No new warnings introduced
• Code follows project conventions

Ruff check and format are clean on both changed files.

Documentation

• Updated README.md (if needed)
• Updated CLAUDE.md (if architecture changed)
• Updated CHANGELOG.md
• Added/updated docstrings
• Updated type hints

N/A — no user-facing API or architecture change.

Package Affected

• taskdog-core

Breaking Changes

• This PR includes breaking changes
• Migration guide provided (if breaking changes)

Checklist

• My code follows the coding standards of this project
• I have performed a self-review of my own code
• I have commented my code in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published

Additional Notes

The issue offered two directions; this takes the smaller one (bool(notes_content) in the use case) rather than changing delete_notes to drop the row, since the repository layer already treats empty content as "no notes".

[Kohei-Wada]

Thanks for the fix! Verified that the change aligns the use case with the repository's content-based notes semantics, and the regression test properly covers the delete-then-detail path. Much appreciated!