Per Ark0N's review on PR #453: rename the design spec to
docs/split-pane-sessions-plan.md, matching every other feature's
*-plan.md convention, and drop the 957-line implementation task plan
(docs/superpowers/plans/2026-09-15-split-pane-sessions.md) — workflow
scaffolding for the subagent-driven-development run, not repo
documentation. Fixes the now-dangling link in architecture-invariants.md.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
