feat(todo): migrate todo form to stable signal forms API (create-only)

[chrisjwalk]

Context

The #213 session migrated libs/todo/src/lib/components/todo-form/todo-form.ts from the compat API (SignalFormControl) to Angular 22.1's stable signal forms (form() + FormRoot/FormField), bundled with larger server-side CRUD changes that are being reverted in favor of a docs-only PR. This issue keeps just the form migration.

The migrated form code lives on the feat/todo-crud-recipes-skills-213 branch (PR #214, commit 48f6740) — salvage it from there.

Scope

• Migrate the create-only todo form to the stable form() API (no edit flow — that stays reverted)
• Wire it into the existing in-memory CRUD API and store (create: rxMethod) — no other UI changes
• Keep spec coverage at the 80% per-file gate
• Update docs/recipes/signal-forms.md to describe the stable API once landed

Notes

• docs/recipes/signal-forms.md currently documents the compat-API pattern (post-revert state)
• Material 22 interop: mat-error relies on explicit @if (touched && invalid) guards (no NgControl bridge for signal forms)