test: add unit tests for core task operations

[LudoLoops]

Issue

No unit tests exist for critical functions like:

• CreateTask
• ReadTaskFile / WriteTaskFile
• GetTasks
• MarkTaskDone
• Task YAML parsing/serialization

Problem

• Hard to verify changes don't break existing functionality
• Impossible to test edge cases (invalid YAML, missing files, etc.)
• Refactoring is risky
• No regression detection

Solution

Create task_test.go with tests for:

func TestReadTaskFile(t *testing.T) {
    // Test valid task file
    // Test missing file
    // Test invalid YAML
    // Test malformed frontmatter
}

func TestCreateTask(t *testing.T) {
    // Test basic creation
    // Test duplicate prevention
    // Test with GitHub issue
    // Test special characters in title
}

func TestGetTasks(t *testing.T) {
    // Test empty vault
    // Test multiple projects
    // Test filtering by project
}

Tools

Use Go's built-in testing package + t.TempDir() for isolated tests.

Benefits

• Catch regressions before shipping
• Enable safe refactoring
• Document expected behavior
• Improve code reliability

Impact

🟢 Low - But highly recommended