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/index.md

@@ -14,7 +14,7 @@ Catalog of all wiki pages. One line per page, organized by type. Updated on ever
 
 - [active-platform-decision.md](concepts/active-platform-decision.md) — why `active-platform` is a skill (not a memory entry); why default = Windows; how it's wired into `project-bootstrap`
 - [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`
-- [context7-setup.md](concepts/context7-setup.md) — switched context7 from manual MCP entries to the official plugin; API key now lives in the plugin's `.mcp.json` as `--api-key`; re-apply after `/plugin update`
+- [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)
 - [repo-layout.md](concepts/repo-layout.md) — flat `skills/`, committed `dist/`, bash + PowerShell scripts; install model
 - [wiki-realignment.md](concepts/wiki-realignment.md) — fixing `project-bootstrap` to create the Karpathy-canonical wiki layout