v0.1.2 — cell_type self-check in the companion skill

Documentation release: the companion skill now knows about the one failure mode that its own structure checks cannot see.

Added

• A cell_type self-check in the style section: the first cell and every ## N. heading cell must be markdown. Markdown prose typed as code is executed as Python and dies on the first non-comment line with SyntaxError: invalid character '（' (U+FF08) — a failure reported several cells away from the cell that actually has the wrong type.
• A wrong-type trap in the "after editing" checklist: after any layout refactor, re-check cell types with notebook_list_cells. notebook_validate only validates structure, so a markdown body sitting in a code cell passes it and still breaks Run All on the first cell.

Changed

• The troubleshooting order for "the notebook won't run" is now: check cell_type first, then check for a cell stuck waiting on input().

No runtime code changed — lib/ is byte-identical to v0.1.1. 61 package tests pass.