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>

skills/using-context7/SKILL.md

@@ -11,6 +11,10 @@ description: Use when answering questions about a specific library, framework, S
 
 Your training data has a cutoff. Library APIs change. If a question names a library, **reach for context7 before answering from memory**, even for libraries you "know" — your recall may be one or two majors behind.
 
+## Prerequisites
+
+This skill assumes `mcp__context7__resolve-library-id` and `mcp__context7__query-docs` are available. If they aren't (the tools are missing from the session, or calls fail with a connection error), the context7 MCP server isn't running for this session. Trigger the **`setup-context7`** skill to install/configure the official plugin (`context7@claude-plugins-official`) and inject the user's API key. It's a one-time procedure with confirmation gates.
+
 ## When to use
 
 Use when the user asks about any of these in the context of a specific library: