Remove stale viewerv2 and skills path references#

Problem#

Two files contained stale references to the removed viewerv2/ phase and old .claude/skills/ path that no longer reflects the current repo structure.

Task / Link#

Closes #1543

Changes#

• scripts/setup/rollback.sh: Replace "ViewerV2" with "(removed)" in phase_names array to preserve 8-element index alignment
• plan.md: Update skills path reference — skills now live in the agent-core repo, not this repository

Notes#

Array length in rollback.sh remains 8 to preserve phase index alignment (0-7).

Testing#

• Verified zero viewerv2 hits outside tasks/ and .git/ after change
• Verified zero .claude/skills hits in .md files outside tasks/ and .git/ after change