feat: publish HoloLake model-native living system source

product-source/hololake-platform/docs/adr/0003-single-note-model.md

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+---
+type: ADR
+id: "0003"
+title: "Single note open at a time (no tabs)"
+status: active
+date: 2026-03-24
+---
+
+## Context
+
+The app originally had a tab bar allowing multiple notes to be open simultaneously (similar to a browser or code editor). After building and shipping it, the tab model was found to add significant UI complexity, state management overhead, and confusion — without a proportional benefit for a notes app.
+
+## Decision
+
+**Remove the tab bar. Only one note is open at a time.** Navigation history (Back/Forward with Cmd+[/]) replaces tabs for moving between recently visited notes. Closed tab history and `useTabManagement` are removed.
+
+## Alternatives considered
+
+- **Keep tabs**: familiar UX, allows comparing notes side by side. Rejected — adds ~2000 lines of complexity, confusing state (which tab is "active"?), and breaks the "editor is sacred" principle.
+- **Tabs + single-note toggle**: configurable per user. Rejected — doubles the state surface and testing burden.
+- **Split pane (two notes at once)**: useful for reference. Deferred — can be added later without tabs, via a dedicated split layout.
+
+## Consequences
+
+- Removes ~2000 lines of code (`TabBar`, `useClosedTabHistory`, `useEditorTabSwap`, `tabLayout`)
+- `handleSelectNote` replaces the current note instead of adding a tab
+- Cmd+W (close tab) and Cmd+Shift+T (reopen closed tab) removed from shortcuts
+- Back/Forward navigation (Cmd+[/Cmd+]) preserves history without tab state
+- Significant simplification of `App.tsx` and editor state
+- Triggers re-evaluation if: multi-note workflows become a top user request