Sync Upstream — merge compound-engineering-plugin into tunan

The local tunan plugin is a rebranded fork of everyinc/compound-engineering-plugin. This skill brings the latest upstream skill changes into the local fork, one merge at a time, preserving every deliberate local customization and the tunan branding.

Skill changes only. Upstream release chores — version bumps, CHANGELOG.md, package.json, the .release-please-manifest.json, manifest version fields, and the upstream-specific top-level README.md — are never ported; the local fork manages its own version and README.

Phase 0: Preconditions

Confirm the working context before touching anything: