feat(cards): add 'note' card type for readable markdown content

New card type renders body as styled markdown with no copy/fill/run
affordance. Glyph: ¶, color: --note.

Migration uses transaction to safely rebuild table constraint.
Checks both 'note' presence and modified_at column to catch
partial migration state.

web/index.html

@@ -64,6 +64,11 @@
                     <span class="type-name">decision</span>
                     <span class="type-hint">record a choice + rationale</span>
                 </button>
+                <button data-type="note" class="type-btn">
+                    <span class="type-glyph glyph-note">¶</span>
+                    <span class="type-name">note</span>
+                    <span class="type-hint">readable markdown content</span>
+                </button>
                 <button data-type="link" class="type-btn">
                     <span class="type-glyph glyph-link">↗</span>
                     <span class="type-name">link</span>