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>

src/scribe/mcp/server.py

@@ -151,11 +151,10 @@ Plans are tasks with kind=plan, and Scribe is the canonical home for them.
 When you begin non-trivial work, call start_planning(project_id, title) FIRST —
 before any brainstorming, design, or plan-writing skill runs. start_planning
 seeds the plan body, returns the project's applicable_rules, and gives you the
-task id you'll write into. If a skill or habit tells you to save a plan or spec
-to `docs/superpowers/plans/*.md` or `docs/superpowers/specs/*.md`, that path is
-superseded here: put the spec/plan content in the kind=plan task's body via
-update_task, and record progress with add_task_log. Local .md files are not
-the record — the task is.
+task id you'll write into. If a habit tells you to save a plan or spec to a local
+`.md` file, that's superseded here: put the spec/plan content in the kind=plan
+task's body via update_task, and record progress with add_task_log. Local .md
+files are not the record — the task is.
 
 Deletes are recoverable: every delete_* tool moves the entity (and its
 descendants) to the trash and returns a deleted_batch_id. Use list_trash() to