docs(install): .wiki concept page for install-cross-platform parity

Closes 3/3 acceptance of [install-ps1]:
- (a) install.ps1 existed
- (b) --prune flag shipped in 6cf0e98
- (c) concept page now written

.wiki/concepts/install-cross-platform.md captures:
- why two scripts (PS for native-Win, sh for Linux/Mac)
- parity contract (shared invariants, flag-naming convention)
- --prune design choices and rejected alternatives
- scope boundary (install-side only; dist/.skill prune is separate)

Index + log updated per .wiki/CLAUDE.md schema.

STATUS.md: [install-ps1] ⚪ → 🟢, new ⚪ task
[install-ps1-build-prune-followup] filed for the analogous
flag on build.{sh,ps1}.

.tasks/STATUS.md

@@ -22,13 +22,21 @@ _Updated: 2026-05-25 (board-cleanup: done-блоки → .archive/done-2026-05.m
 
 ---
 
-## ⚪ [install-ps1] — paired install.sh + install.ps1 (cross-platform parity, with --prune)
-**Status:** ready
-**Where I stopped:** Verify 2026-05-07 (factory-context audit): (a) ✅ `scripts/install.ps1` существует — кросс-платформа есть; (b) ❌ `--prune` flag отсутствует в обоих скриптах; (c) ❌ doc `.wiki/concepts/install-cross-platform.md` не написан. 1/3 acceptance — не закрывать.
+## 🟢 [install-ps1] — paired install.sh + install.ps1 (cross-platform parity, with --prune)
+**Status:** done
+**Where I stopped:** All 3 acceptance items shipped 2026-05-25. (a) `scripts/install.ps1` existed pre-task. (b) `--prune` / `-Prune` flag added in commit `6cf0e98` (paired sh + ps1; combined-flag pattern; global-scan; default off; smoke-tested on disposable target dirs against fake stale skills). (c) Concept page `.wiki/concepts/install-cross-platform.md` written this commit: parity contract, --prune design choices, rejected alternatives, scope vs build/.skill prune.
+**Next action:** (none — kept until merged)
+**Branch:** master
+<!-- closed-by: vitya@DESKTOP-NSEF0UK / 2026-05-25 / 3/3 acceptance met; build-script analogue for dist/.skill prune remains separate concern -->
 
-Пересечение с `[claude-skills-update-skill]`: оба про install/build/prune trinity. Имеет смысл сделать вместе — `--prune` логику добавить в существующие `install.{ps1,sh}` параллельно с новыми `update.{ps1,sh}` (которые будут жить в том же `scripts/`). Doc `install-cross-platform.md` тогда покрывает всю четвёрку.
-**Next action:** (a) уже есть. Остаётся: (b) добавить `--prune` flag в `scripts/install.ps1` и `scripts/install.sh` — для каждого имени в `~/.claude/skills/<name>/`, отсутствующего в `skills/<name>/`, удалить (lesson из `[compress-dedup]`). (c) написать `.wiki/concepts/install-cross-platform.md` — документировать parity-инвариант (PS+bash сpan, identical behavior, paired flags, error semantics). Опционально: совместить с `[claude-skills-update-skill]` (см. её next_action).
+---
+
+## ⚪ [install-ps1-build-prune-followup] — Add matching `--prune` flag to `scripts/build.{sh,ps1}` for `dist/<name>.skill` artefacts (analogue of install-script prune).
+**Status:** ready
+**Where I stopped:** Surfaced 2026-05-25 from `[install-ps1]` close — install-side prune ships, but `dist/<name>.skill` artefacts have the same staleness problem (e.g. `dist/using-synology-ops.skill` would linger if it had ever been built). Currently no .skill exists for the retired skill, but the gap is real for future renames/retires.
+**Next action:** Add `--prune` to `scripts/build.sh` and `scripts/build.ps1`: scan `dist/*.skill`, drop any whose `<name>` is not in `skills/`. Same default-off / print-and-delete pattern as install-side. Update `.wiki/concepts/install-cross-platform.md` to extend the parity contract to build scripts.
 **Branch:** (not started)
+<!-- created-by: vitya@DESKTOP-NSEF0UK / 2026-05-25 / origin: [install-ps1] close — natural extension to build scripts -->
 
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