feat(build): add --prune flag to build.{sh,ps1} [skip-tdd: wrapper]

Symmetric to install-side --prune (e871c20). Removes
dist/<name>.skill files whose <name> is not in skills/*.

Design choices match install:
- Combined flag (build + prune in one run)
- Global scan, ignores -Names / positional filter
- Default off, print-and-delete, no confirmation

Bash wrinkle: when build.sh delegates to powershell.exe -File
build.ps1 (Windows-without-zip case), --prune is NOT forwarded.
Bash runs prune itself at the end of the script against the
shared dist/. Keeps the delegation surface narrow and the prune
logic single-sourced per shell.

[skip-tdd: wrapper] — same carve-out as install-side, smoke-test
evidence: fake dist/fake-stale-{sh,ps}.skill files created in real
dist/, ran build.{sh,ps1} --prune, verified fakes removed, real
caveman.skill etc. left intact.

Wiki .wiki/concepts/install-cross-platform.md extended:
- title broadened (Install → Install / Build)
- new "Build-side --prune" section
- scope note: dist-hermes/ is separate (managed by build-hermes.py)

Closes [install-ps1-build-prune-followup].

.wiki/index.md

@@ -22,7 +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-cross-platform.md](concepts/install-cross-platform.md) — paired-script parity contract for `install.{ps1,sh}` AND `build.{ps1,sh}`; rationale for the `--prune` / `-Prune` flag (combined-with-action, global-scan, default-off); install-side prunes target dirs, build-side prunes `dist/*.skill` files
 - [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