feat(skills): add stacked-pr-wizard workflow skill

[thekidnamedkd]

Description

Adds the stacked-pr-wizard workflow skill to the canonical skills/shared/ tree. Stacked on top of #1275 (the skills refactor that establishes skills/), so it lands at its new home once the refactor merges.

The skill packages the gh stack (github/gh-stack) workflow for composing and managing stacked pull requests — greenfield stacks and retroactive decomposition of a monolithic branch into reviewable layers.

What changed:

• skills/shared/stacked-pr-wizard/SKILL.md — portable frontmatter + core workflow (greenfield, retroactive decomposition, shared-file strategy, changeset choreography, teardown guardrails, force-push discipline).
• skills/shared/stacked-pr-wizard/references/gh-stack-commands.md — full gh stack command reference loaded on demand.

Repo-specific quirks baked in: multi-remote worktree fix (git config remote.pushDefault origin), type(scope): summary commit convention, @aragon/app none-bump changeset pattern, and the one-changeset-per-release-scope rule.

Verification:

• node --test scripts/sync-skills.test.mjs — 13/13 pass (frontmatter, name/dir match, uniqueness, no empty dirs, generated-root flattening + gitignore contract).
• node scripts/sync-skills.mjs (CI unset) — installs stacked-pr-wizard into .agents/skills/ and .claude/skills/ (8 skills total).

Type of Change

• Minor: Feature (non-breaking change which adds new functionality)

Part of stack #1300. Depends on #1275 — merge bottom-up.