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/log.md

@@ -61,3 +61,5 @@ Parseable: `grep "^## \[" .wiki/log.md | tail -20`.
 ## [2026-05-22] ingest | concepts/interns-grep-audit-design
 
 ## [2026-05-25] decision | session-handoff-skill-design — design rationale for the `session-handoff` skill captured in wiki after cluster 7/7 closure; sliding overwrite of `.tasks/NEXT_SESSION.md`, phrase whitelist + substantive-commit heuristic, opt-in PostToolUse hook, orient+ask default, source: `~/projects/.workshop/.archive/2026-05-24-session-handoff-skill.md` Round 1 + Round 2
+
+## [2026-05-25] decision | install-cross-platform — `install.{ps1,sh}` paired-script parity contract documented; `--prune` / `-Prune` flag rationale (combined-with-install, global-scan ignores names filter, default-off, print-and-delete no prompt); shipped in commit `6cf0e98` with `[skip-tdd: wrapper]` carve-out + smoke-test evidence; closes 2/3 of `[install-ps1]` acceptance (the doc + flag), `dist/`-prune analogue deferred to `build` scripts