docs: switch context7 to official plugin; remove three manual MCP entries

Context: user previously had context7 MCP server registered manually in
three places (~/.claude/settings.json + two in ~/.claude.json), with the
CONTEXT7_API_KEY in HTTP-header form. Replaced with the official
context7@claude-plugins-official plugin.

What changed (outside the repo):
- ~/.claude/plugins/cache/.../context7/unknown/.mcp.json — added
  "--api-key" CLI flag to args (Upstash docs recommend this for stdio).
- ~/.claude/settings.json — removed top-level mcpServers.context7.
- ~/.claude.json — removed top-level mcpServers.context7 and a legacy
  project-scoped entry under projects[snolla-admin-ui].

Backups saved with suffix .bak-20260428-114532.

What changed in the repo (this commit):
- .wiki/concepts/context7-setup.md — new page documenting where the key
  lives, the /plugin update gotcha, and the rollback path.
- .wiki/index.md, log.md — linked / logged.
- .tasks/STATUS.md — task moved to Done.

Plugin takes effect after Claude Code restart; current session still uses
the legacy HTTP transport (verified via smoke test on resolve-library-id).

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

.wiki/log.md

@@ -18,3 +18,4 @@ Parseable: `grep "^## \[" .wiki/log.md | tail -20`.
 ## [2026-04-28] decision | active-platform — skill chosen over global CLAUDE.md / project memory; default Windows; wired into project-bootstrap
 ## [2026-04-28] refactor | wiki-realignment — fixed project-bootstrap Step 3 to create Karpathy-canonical layout
 ## [2026-04-28] refactor | this repo's `.wiki/` migrated to canonical layout (SUMMARY.md→index.md, source/→concepts/, added log.md/overview.md/CLAUDE.md schema, raw/README.md)
+## [2026-04-28] decision | context7-setup — switched to official plugin; --api-key injected into plugin's .mcp.json; three manual MCP entries removed