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/verification/SKILL.md

@@ -0,0 +1,32 @@
+---
+name: verification
+description: Use before claiming a task is done or a change works — confirm it actually does, then record that you did. Triggers when you're about to report completion, mark a task done, or say "it works" / "fixed". Guards against declaring success on unverified work.
+---
+
+# Verification before completion
+
+"Done" means verified, not "written." Before you claim a change works or set a
+task `done`, confirm it against reality and record what you checked.
+
+## Verify against reality
+
+- Exercise the actual behavior — run it, test it, observe the output. Prefer the
+  real path over "it should work by inspection."
+- Check the thing the user actually asked for, not a proxy for it.
+- If you *can't* verify something (no environment, needs hardware, needs the
+  operator), say so explicitly — name what's unverified rather than letting it
+  read as passed.
+
+## Record the result, then close
+
+- Log what you verified, and how, to the task with `add_task_log` — the check is
+  part of the record, not a private step.
+- Only then set the task `done`. Never mark finished work you haven't confirmed,
+  and never leave confirmed work sitting at `in_progress`.
+- If verification surfaced a problem, capture it (tag `issue`) and keep the task
+  open — a found problem is a pivot to record, not something to quietly skip.
+
+## Honesty over optimism
+
+A truthful "verified X; could not verify Y" is worth more than a confident
+"done." The record is only useful if `done` reliably means done.