fix(skills): keep Pi design workflows single-model

[zhanfeng-alpha]

Why

Keep Pi design work single-model and parent-owned. Architect now starts with one sketch, permits an optional parent pick, and proceeds to implementation without arena, competing runners, or cross-judging.

Supersedes #12. A judge-brief rule does not address the workflow that creates the judge.

Scope

• Add Pi-owned replacements for architect and Exhaust the Design Space, plus exact-match poteto-mode routing patches.
• Apply overlays through conform-skills.py on every install and refresh. Point the sticky prompt and /poteto at the installed copy.
• Preserve same-model parallel work for disjoint workstreams. The adapter's policy overrides cross-model defaults in other imported skills.
• No upstream pstack edits, live host changes, or subagent launches. Other imported workflows are not rewritten.

Tradeoffs

Upstream changes to a patch target stop installation with an explicit drift error. They require review rather than silently restoring old routing.

Verification

• All 10 README commands passed with an isolated HOME, including 18 existing Python tests and five new overlay tests.
• The new tests exercise actual installation and update-pstack apply, legacy-copy migration, repeat-apply idempotence, source preservation, symlink safety, and missing or duplicate patch targets.
• A temporary install from real pstack passed Pi loader checks for all three skills. Reinstallation preserved installed bytes and source hashes. Current cursor/plugins main matched every patch target.
• The existing live-HOME check stops because the host still has retired @narumitw/pi-goal installed. This PR does not refresh the host or remove packages.