feat(demo): add VHS demo tape and generated GIF

Create docs/demo.tape scripting a ~15s pivot axis reassignment demo.
Hide shell startup and quit so the GIF opens directly on the TUI.
Set up git-lfs for docs/*.gif. Merge LFS hooks with existing beads hooks.

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

docs/demo.tape

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+# improvise demo — pivot axis reassignment
+# Run: nix develop --command vhs docs/demo.tape
+
+Output docs/demo.gif
+
+Set FontSize 14
+Set Width 1200
+Set Height 650
+Set Theme "Dracula"
+
+# Hide the shell prompt and startup
+Hide
+Type "./result/bin/improvise examples/demo.improv"
+Enter
+Sleep 2s
+Show
+
+# Show the initial pivot: Product×Region on rows, Date_Month×Measure on columns
+Sleep 2s
+
+# Enter tile mode
+Type "T"
+Sleep 1s
+
+# Navigate to Region tile (index 5: _Index, _Dim, Customer, Date, Product, Region)
+Type "lllll"
+Sleep 800ms
+
+# Move Region from Row to Column axis
+Type "c"
+Sleep 2s
+
+# Navigate back to Customer tile (index 2)
+Type "hhh"
+Sleep 600ms
+
+# Move Customer from Page to Row axis
+Type "r"
+Sleep 2s
+
+# Exit tile mode
+Escape
+Sleep 1500ms
+
+# Transpose to show another arrangement
+Type "t"
+Sleep 2s
+
+# Quit without recording the exit
+Hide
+Type ":q"
+Enter
+Sleep 500ms