feat: add DELETE /tasks/:id endpoint

[greatwallconnectblitzlog]

Closes #1

Add the missing DELETE endpoint that closes the basic CRUD surface.

• src/store.rs — add remove(&mut self, id: &Uuid) -> bool.
• src/handlers.rs — add delete_task handler returning 204 or 404.
• src/bin/taskforge.rs — wire up .delete(handlers::delete_task).
• tests/integration.rs — cover both success and not-found paths.

Verification:

• cargo test — 6/6 pass.
• cargo fmt --check && cargo clippy --all-targets -- -D warnings — clean.