034 — Move Focus/Task invariants into domain

[archae0pteryx]

What to build

Two domain rules currently live in crates/commands/src/focus.rs:

• title.trim().is_empty() check in Commands::create_focus
• text.trim().is_empty() check in Commands::append_task

These are domain invariants that belong in crates/domain/. Move them there as typed constructors. Commands drops its own validation guards.

Domain changes (crates/domain/)

impl NewFocus {
    pub fn new(title: impl Into<String>, description: impl Into<String>) -> Result<Self, DomainError>
    // Err if title.trim().is_empty()
}

pub struct TaskText(String);
impl TaskText {
    pub fn new(text: impl Into<String>) -> Result<Self, DomainError>
    // Err if text.trim().is_empty()
    pub fn as_str(&self) -> &str
}

#[derive(Debug, thiserror::Error)]
pub enum DomainError {
    #[error("title must not be empty")]
    EmptyTitle,
    #[error("task text must not be empty")]
    EmptyTaskText,
}

Commands changes

Remove the trim().is_empty() guards in Commands::create_focus and Commands::append_task. Use NewFocus::new(...) and TaskText::new(...). Map DomainError to CommandError::BadRequest.

Domain tests own the invariant assertions. Commands tests drop the validation assertions.

Completion promise

Focus title and task text invariants are enforced once in crates/domain/ via typed constructors; no validation logic remains in the commands layer.

Acceptance criteria

• NewFocus::new returns Err(DomainError::EmptyTitle) for blank title
• TaskText::new returns Err(DomainError::EmptyTaskText) for blank text
• Domain has tests for both error cases
• Commands::create_focus and Commands::append_task contain no trim().is_empty() guards
• Existing acceptance criteria for create_focus and append_task commands still pass end-to-end
• task check green

Blocked by

None — can start immediately.

[archae0pteryx]

Resolved by PR #40 (034 focus/task invariants in domain).