feat: setup-context7 skill + using-context7 prerequisites pointer

Split context7 concerns into two skills:
- using-context7 (policy, every-time): when to call, how to query well
- setup-context7 (infrastructure, one-time): install plugin, inject API
  key, clean manual MCP entries, with confirmation gates

setup-context7 reuses an existing CONTEXT7_API_KEY by searching, in order:
~/.claude/settings.json → ~/.claude.json (top-level + project-scoped) →
~/.claude/settings.local.json → env. Asks the user only if nothing found.
Cross-platform (paths identical on Win/Linux/macOS; only JSON validator
differs). Phase-2 confirmation gate before any write; per-edit JSON
validation; rollback procedure documented.

using-context7 gets a small Prerequisites section pointing at
setup-context7 when MCP tools are missing.

Bonus fix: scripts/build.sh PS multi-arg bug. Comma-joined -Names didn't
split into [string[]] when build.ps1 was invoked via -File. Now the bash
side loops one PS invocation per skill.

Wiki: extended .wiki/concepts/context7-setup.md with the design rationale
(why split policy from setup) — template for future "X plugin + how to
use X" pairs. index.md and log.md updated.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scripts/build.sh

@@ -36,14 +36,16 @@ if command -v zip >/dev/null 2>&1; then
   done
 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.
   ps1="$SCRIPT_DIR/build.ps1"
+  ps1_win="$(cygpath -w "$ps1" 2>/dev/null || echo "$ps1")"
   if [ "$#" -eq 0 ]; then
-    powershell.exe -NoProfile -ExecutionPolicy Bypass -File "$(cygpath -w "$ps1" 2>/dev/null || echo "$ps1")"
+    powershell.exe -NoProfile -ExecutionPolicy Bypass -File "$ps1_win"
   else
-    # PS array via -File: comma-separated, no extra quoting (bash arg becomes one PS token).
-    joined=$(printf ",%s" "$@")
-    joined="${joined:1}"
-    powershell.exe -NoProfile -ExecutionPolicy Bypass -File "$(cygpath -w "$ps1" 2>/dev/null || echo "$ps1")" -Names "$joined"
+    for name in "$@"; do
+      powershell.exe -NoProfile -ExecutionPolicy Bypass -File "$ps1_win" -Names "$name"
+    done
   fi
 else
   echo "error: need either 'zip' (Linux/macOS) or PowerShell (Windows) to build .skill archives" >&2