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].

scripts/build.sh

@@ -1,6 +1,10 @@
 #!/usr/bin/env bash
 # Build .skill archives from skills/<name>/ into dist/<name>.skill
-# Usage: build.sh [name...]   (no args = all)
+# Usage: build.sh [--prune] [name...]
+#   no args        = build all skills/* into dist/<name>.skill
+#   --prune        = after build, remove dist/<name>.skill files whose <name>
+#                    is NOT in skills/* (analogue of install.sh --prune).
+#                    Prune always scans the full dist/, ignores name filter.
 #
 # Uses `zip` if available; otherwise delegates to scripts/build.ps1
 # (so the script works on Linux, macOS, and Windows-with-git-bash without
@@ -12,9 +16,18 @@ ROOT="$(cd "$SCRIPT_DIR/.." && pwd)"
 SRC="$ROOT/skills"
 DIST="$ROOT/dist"
 
+prune=0
+positional=()
+for arg in "$@"; do
+  case "$arg" in
+    --prune) prune=1 ;;
+    *) positional+=("$arg") ;;
+  esac
+done
+
 if command -v zip >/dev/null 2>&1; then
   mkdir -p "$DIST"
-  if [ "$#" -eq 0 ]; then
+  if [ "${#positional[@]}" -eq 0 ]; then
     # Portable across bash 3.2 (stock macOS) and bash 4+ (Linux, git-bash):
     # avoid `mapfile` (bash 4+) and `find -printf` (GNU find only).
     names=()
@@ -25,7 +38,7 @@ if command -v zip >/dev/null 2>&1; then
     IFS=$'\n' names=($(printf '%s\n' "${names[@]}" | sort))
     unset IFS
   else
-    names=("$@")
+    names=("${positional[@]}")
   fi
   for name in "${names[@]}"; do
     src_dir="$SRC/$name"
@@ -46,12 +59,14 @@ elif command -v powershell.exe >/dev/null 2>&1; then
   # Windows fallback: delegate to build.ps1 (proper ZIP via .NET API).
   # PS array binding via -File is fragile (commas don't always split into [string[]]),
   # so call build.ps1 once per skill and let the bash loop do the work.
+  # --prune is NOT forwarded — bash handles prune at the end of this script
+  # against the same dist/, regardless of which build path ran.
   ps1="$SCRIPT_DIR/build.ps1"
   ps1_win="$(cygpath -w "$ps1" 2>/dev/null || echo "$ps1")"
-  if [ "$#" -eq 0 ]; then
+  if [ "${#positional[@]}" -eq 0 ]; then
     powershell.exe -NoProfile -ExecutionPolicy Bypass -File "$ps1_win"
   else
-    for name in "$@"; do
+    for name in "${positional[@]}"; do
       powershell.exe -NoProfile -ExecutionPolicy Bypass -File "$ps1_win" -Names "$name"
     done
   fi
@@ -59,3 +74,14 @@ else
   echo "error: need either 'zip' (Linux/macOS) or PowerShell (Windows) to build .skill archives" >&2
   exit 1
 fi
+
+if [ "$prune" -eq 1 ]; then
+  for f in "$DIST"/*.skill; do
+    [ -f "$f" ] || continue
+    name="$(basename "$f" .skill)"
+    if [ ! -d "$SRC/$name" ]; then
+      echo "pruning: $name (not in skills/) → $f"
+      rm -f "$f"
+    fi
+  done
+fi