Strengthen using-superpowers for explicit skill requests

Addresses failure mode where Claude skips skill invocation even when
user explicitly requests it by name (e.g., "subagent-driven-development,
please").

Skill changes:
- "Check for skills" → "Invoke relevant or requested skills"
- "BEFORE ANY RESPONSE" → "BEFORE any response or action"
- Added reassurance that wrong skill invocations are okay
- New red flag: "I know what that means"

Tests:
- New test suite for explicit skill requests
- Single-turn and multi-turn scenarios with --continue
- Tests with haiku model and user CLAUDE.md

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

@@ -0,0 +1,11 @@
+[Previous assistant message]:
+Plan complete and saved to docs/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
+2. Parallel Session (separate) - Open a new Claude Code session with the execute-plan skill, batch execution with review checkpoints
+
+Which approach do you want to use for implementation?
+
+[Your response]:
+subagent-driven-development, please