Add a project-local verification skill for Open Pstack

[arjitj2]

Add a Codex project skill with a maintained feature map for installed skill invocation, model setup, and the shipped CLI tools. Ground the commands in this checkout, run one mapped feature end to end in isolated state, and retain action/output evidence after cleanup. Keep local CLI proof distinct from the installed-harness release gate.

[ericlitman]

Reopening. There's no project verification skill on main (checked at 1c91e65) or in the maintainer checkout. Factory build sandboxes don't have bun, claude, codex, or grok, so the installed-candidate gate in AGENTS.md has to run on the Mac.

Acceptance criteria:

• A project skill installs a PR head into Claude Code and Codex, runs the changed behavior from each real surface, and writes the PR template's live-evidence block (installed version, surface, action, observed result).
• The skill is the step that marks a draft PR ready. Mergify queues only non-draft PRs (Configure Mergify to auto-queue PRs once verify and Unfret pass #100, ci: queue PRs in Mergify once verify and Unfret pass #101), so the live verdict comes before the queue.
• Local CLI proof stays separate from the installed-harness gate, as the original scope said.