refactor: restructure specs and plans directories

- Specs (brainstorming output) now go to docs/superpowers/specs/
- Plans (writing-plans output) now go to docs/superpowers/plans/
- User preferences for locations override these defaults
- Update all skill references and test files

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/explicit-skill-requests/prompts/action-oriented.txt

@@ -1,3 +1,3 @@
-The plan is done. docs/plans/auth-system.md has everything.
+The plan is done. docs/superpowers/plans/auth-system.md has everything.
 
 Do subagent-driven development on this - start with Task 1, dispatch a subagent, then we'll review.

tests/explicit-skill-requests/prompts/after-planning-flow.txt

@@ -1,4 +1,4 @@
-Great, the plan is complete. I've saved it to docs/plans/auth-system.md.
+Great, the plan is complete. I've saved it to docs/superpowers/plans/auth-system.md.
 
 Here's a summary of what we designed:
 - Task 1: Add User Model with email/password fields

tests/explicit-skill-requests/prompts/claude-suggested-it.txt

@@ -1,5 +1,5 @@
 [Previous assistant message]:
-Plan complete and saved to docs/plans/auth-system.md.
+Plan complete and saved to docs/superpowers/plans/auth-system.md.
 
 Two execution options:
 1. Subagent-Driven (this session) - I dispatch a fresh subagent per task, review between tasks, fast iteration within this conversation

tests/explicit-skill-requests/prompts/i-know-what-sdd-means.txt

@@ -1,4 +1,4 @@
-I have my implementation plan ready at docs/plans/auth-system.md.
+I have my implementation plan ready at docs/superpowers/plans/auth-system.md.
 
 I want to use subagent-driven-development to execute it. That means:
 - Dispatch a fresh subagent for each task in the plan

tests/explicit-skill-requests/prompts/mid-conversation-execute-plan.txt

@@ -1,3 +1,3 @@
-I have a plan at docs/plans/auth-system.md that's ready to implement.
+I have a plan at docs/superpowers/plans/auth-system.md that's ready to implement.
 
 subagent-driven-development, please

tests/explicit-skill-requests/prompts/skip-formalities.txt

@@ -1,3 +1,3 @@
-Plan is at docs/plans/auth-system.md.
+Plan is at docs/superpowers/plans/auth-system.md.
 
 subagent-driven-development, please. Don't waste time - just read the plan and start dispatching subagents immediately.