feat: add active-platform skill (Win/Linux/macOS switcher)

- skills/active-platform/SKILL.md: default = Windows/PowerShell; trigger
  phrases in RU+EN switch the active platform for the session; per-platform
  conventions (chaining, paths, env vars, install scripts) documented
- dist/active-platform.skill: built archive
- .wiki/source/active-platform-decision.md: rationale (skill chosen over
  global CLAUDE.md / project memory; user wanted a discoverable, iterable
  artifact)
- scripts/build.sh: fix PS arg-passing — single-quotes were leaking into
  -Names value, so multi-arg builds on Windows were rejected with
  "not found in skills/"

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.wiki/SUMMARY.md

@@ -7,3 +7,4 @@ The agent updates this file whenever source/ changes.
 
 - [repo-layout.md](source/repo-layout.md) — how the repo is organized: `skills/` sources, `dist/` builds, `scripts/` build & install
 - [build-notes.md](source/build-notes.md) — why `build.ps1` exists, ZIP layout, PowerShell 5.1 backslash gotcha
+- [active-platform-decision.md](source/active-platform-decision.md) — why we made `active-platform` a skill instead of a memory entry