docs(skill): add upstream fork sync workflow

[aeei]

Summary

• add repository-owned .pi/skills/sync-upstream-fork
• encode the canonical main + exact upstream release tag merge policy
• preserve AEEI/upstream path ownership and lockfile regeneration rules
• enforce immutable package/archive and trusted-publishing gates
• include a read-only divergence/merge-tree analyzer

Current baseline

• upstream latest release: v5.1.3
• upstream main: d5af4f22e951712e084df8b7c513a1708e56d372
• downstream main: 2a8ac3e796e4b92341f633dd1d2bbe63a6ad821b
• result: upstream behind 0; current latest release fully integrated

Validation

• Agent Skills frontmatter/name/path/reference contract
• bash -n and shellcheck
• analyzer dry-run against origin/main and v5.1.3
• git diff --check