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/skills/brainstorming/SKILL.md

@@ -0,0 +1,34 @@
+---
+name: brainstorming
+description: Use when exploring options or shaping a direction before committing — open up the solution space instead of jumping to the first idea. Triggers on "how should we approach X", "what are the options", weighing trade-offs, or any open-ended design question. Recall prior thinking first; capture the decision after.
+---
+
+# Brainstorming
+
+Widen before you narrow. The first idea is rarely the best; the goal is a few
+real options and a reasoned choice — not a single path defended after the fact.
+
+## Recall first
+
+`search` Scribe before generating from scratch — a prior decision, note, or
+brainstorm on this often already exists. Build on it instead of repeating it.
+
+## Open up
+
+- Generate a few genuinely *different* options, not variations of one. Include at
+  least one you don't initially favor.
+- For each: the core idea, what it's good at, and its main cost or risk — briefly.
+- Resist converging until the space is actually explored.
+
+## Then choose
+
+- Recommend one, and say *why* — the trade-off that decided it, not just the pick.
+- Surface the 1–2 places you made an interpretive call, so the operator can
+  redirect before it's baked in.
+
+## Capture the decision
+
+When a direction is chosen, record it in Scribe (`create_note`, e.g. tag
+`decision`): the choice, the alternatives weighed, and the reason. That's what
+keeps the same question from being re-litigated later — and what a future
+session reads to understand *why*, not just *what*.