docs(wiki): skill-vs-plugin concept page; flag task-tracking bootstrap gap

- .wiki/concepts/skill-vs-plugin.md: when a bare SKILL.md suffices vs when
  a plugin is required (slash commands, hooks, sub-agents, MCP via
  marketplace). Concrete breakdown of `superpowers` showing why each of
  its non-skill artifacts needs the plugin format.
- .wiki/index.md, log.md: linked / logged.

Backlog: project-bootstrap Step 4 has the same defect as the wiki step
(now fixed in 1cae0a2) — it creates a single flat STATUS.md and never
mentions the per-task files that task-status-wiki actually expects. Added
to .tasks/STATUS.md backlog for a conscious follow-up.

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

.wiki/log.md

@@ -21,3 +21,4 @@ Parseable: `grep "^## \[" .wiki/log.md | tail -20`.
 ## [2026-04-28] decision | context7-setup — switched to official plugin; --api-key injected into plugin's .mcp.json; three manual MCP entries removed
 ## [2026-04-28] decision | setup-context7 skill — formalized the install/migrate algorithm; using-context7 gets a Prerequisites pointer; build.sh PS multi-arg bug fixed (loop instead of comma-joined -Names)
 ## [2026-04-28] verify | setup-context7 — Vitya ran using-context7 in a session that needed setup; Prerequisites pointer triggered setup-context7; full flow worked end-to-end. Pattern (policy + setup split) validated.
+## [2026-04-28] decision | skill-vs-plugin — documented when a bare skill suffices vs when a plugin is required (slash commands, hooks, sub-agents, MCP via marketplace)