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>

.wiki/concepts/context7-setup.md

@@ -71,3 +71,20 @@ Manual MCP entries in `~/.claude.json` / `settings.json` are easy to:
 - drift from the canonical version
 
 The plugin centralizes the registration and gets versioned through the marketplace. The price is a single edit-after-update for the API key.
+
+## Now captured as a skill
+
+The procedure above is now formalized as the **`setup-context7`** skill ([`skills/setup-context7/SKILL.md`](../../skills/setup-context7/SKILL.md)). It runs the same algorithm with confirmation gates and key-discovery logic (search `settings.json` → `.claude.json` → env, reuse what's there, never invent). `using-context7` got a small **Prerequisites** section pointing at it.
+
+### Why split into two skills
+
+Two distinct concerns:
+
+- **Policy** (every-time, short-running): when to call resolve-library-id, query budget, how to phrase queries — this lives in `using-context7`.
+- **Setup** (one-time, mutates user config): install plugin, inject key, clean manual entries — this lives in `setup-context7`.
+
+Mixing them would make the policy skill ~2× larger, dilute its description (worse triggering), and make every library question pull setup procedure into context. The split is also a template for future "X plugin + how-to-use-X" skill pairs.
+
+### Cross-platform
+
+The setup skill is platform-agnostic. Only the JSON validator differs (PowerShell on Windows, `jq` / Python on Linux/macOS). Paths (`~/.claude/...`) are identical.