Restore spec-kitty.checklist skill package

Deferred from #1310 (mission `01KSMG8Y`, WP09 subtask T041).

The `spec-kitty.checklist` skill package is currently missing from the Agent Skills pipeline. Restoring it requires dedicated template work and skill packaging infrastructure that falls outside the pre-doctrine-stabilization scope.

**What needs to happen:**
- Re-create the checklist skill package under `.agents/skills/spec-kitty.checklist/SKILL.md`
- Register it in `.kittify/command-skills-manifest.json`
- Ensure the 4 Agent Skills agents (Codex, Vibe, Pi, Letta) can invoke it
- Update migration logic to deploy the checklist skill on `spec-kitty upgrade`

Explicitly re-deferred per C-008 of the mission spec.

Start with .agents/skills/spec-kitty.checklist/SKILL.md and .kittify/command-skills-manifest.json, then locate the migration logic used by spec-kitty upgrade. Verify how the four named Agent Skills agents invoke registered skills. Done means the package is recreated, registered, invokable by Codex, Vibe, Pi, and Letta, and deployed by the upgrade command.

Written by the indexing model from the issue text.