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}.

.wiki/index.md

@@ -22,6 +22,7 @@ Catalog of all wiki pages. One line per page, organized by type. Updated on ever
 - [bootstrap-skill-deps-check.md](concepts/bootstrap-skill-deps-check.md) — project-bootstrap@1.7.0 — Step 5.6 collapses the per-skill "detect-and-recommend" mirror shape into one generic `trigger → fulfiller` table walker (skill vs plugin kind, never auto-install); subsumes the deferred `[bootstrap-recommend-projects-meta]` and the existing `superpowers`-only detector
 - [bootstrap-manifest.md](concepts/bootstrap-manifest.md) — record of which `project-bootstrap` / `setup-wiki` / `setup-tasks` versions initialized this project's `.wiki/` and `.tasks/` layout (overwritten on re-bootstrap; history in git)
 - [build-notes.md](concepts/build-notes.md) — why `build.ps1` exists alongside `build.sh`; PS 5.1 backslash-in-zip gotcha; how to extract a `.skill`
+- [install-cross-platform.md](concepts/install-cross-platform.md) — parity contract between `install.ps1` and `install.sh` (paired-script invariant); rationale for `--prune` / `-Prune` flag (combined-with-install, global-scan, default-off)
 - [install-portability.md](concepts/install-portability.md) — `install.sh` / `build.sh` rewritten to drop `mapfile` (bash 4+) and `find -printf` (GNU only) so stock macOS (bash 3.2 + BSD find) works
 - [context7-setup.md](concepts/context7-setup.md) — switched context7 from manual MCP entries to the official plugin; API key in `.mcp.json` as `--api-key`; now also captured as `setup-context7` skill (one-time install/migrate flow with key discovery)
 - [projects-meta-skills.md](concepts/projects-meta-skills.md) — `setup-projects-meta` + `using-projects-meta` skill pair for the local `projects-meta-mcp` stdio server (cross-project tasks + shared Gitea wiki); local-first rule + two-step mutation pattern