chore: remove stale SPEC.md, salvage vision into docs/design-notes.md

Audited SPEC.md against code — mostly accurate but redundant with
repo-map.md and design-principles.md. Minor drift in storage internals,
wizard step count, and mode representation. Salvaged product vision and
non-goals into docs/design-notes.md with staleness disclaimer.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/design-notes.md

@@ -0,0 +1,28 @@
+# Design Notes
+
+> **Staleness warning:** This document captures conceptual framing from the
+> original specification. It is not kept in sync with the code. For current
+> architecture and types, see `context/repo-map.md` and
+> `context/design-principles.md`.
+
+## Product Vision
+
+Traditional spreadsheets conflate data, formulas, and presentation into a single
+flat grid addressed by opaque cell references (A1, B7). This makes models
+fragile, hard to audit, and impossible to rearrange without rewriting formulas.
+
+Improvise treats data as a multi-dimensional, semantically labeled structure --
+separating data, computation, and views into independent layers. Formulas
+reference meaningful names, views can be rearranged instantly, and the same
+dataset can be explored from multiple perspectives simultaneously.
+
+The application compiles to a single static binary and provides a rich TUI
+experience.
+
+## Non-Goals (v1)
+
+- Scripting/macro language beyond the formula system.
+- Collaborative/multi-user editing.
+- Live external data sources (databases, APIs).
+- Charts or graphical visualization.
+- Multi-level undo history.