feat(plugin): add 4 Scribe-native process-skills (restore superpowers gap)

Superpowers was uninstalled but its replacements were never built (only
using-scribe shipped) — a live functional hole. Author the 4 the operator
wants back, each integrated with Scribe's toolset rather than generic copies:
- writing-plans     -> start_planning / kind=plan task, not local .md
- systematic-debugging -> capture issue (symptom->cause->fix, tag issue) on resolve
- verification      -> log results to the task work-log; honest done
- brainstorming     -> recall prior thinking first; capture the decision note

Skipped TDD + receiving-code-review per operator (well-covered by Claude/them).
Manifest + using-scribe list now advertise only the 4 that ship. Remove the
stale docs/superpowers/*.md reference in _INSTRUCTIONS (superpowers is gone).
Plugin 0.1.6 -> 0.1.7.

Refs plan 821 (Phase 3 of 755).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugin/.claude-plugin/plugin.json

@@ -1,7 +1,7 @@
 {
   "name": "scribe",
-  "description": "Scribe second brain for Claude Code: MCP tools over your notes/tasks/projects/rules, a session-start push channel that surfaces your always-on rules + active-project context, and a set of universal process-skills (brainstorm, debug, TDD, plan, verify). Replaces superpowers + file-memory with one app-backed plugin.",
-  "version": "0.1.6",
+  "description": "Scribe second brain for Claude Code: MCP tools over your notes/tasks/projects/rules, a session-start push channel that surfaces your always-on rules + active-project context, and a set of process-skills (writing-plans, systematic-debugging, verification, brainstorming). Replaces superpowers + file-memory with one app-backed plugin.",
+  "version": "0.1.7",
   "author": { "name": "Bryan Van Deusen" },
   "mcpServers": {
     "scribe": {