Build the template chooser and Save as Template

The chooser completes its three tiers: bundled by template order, then
keyed user templates, then keyless boards by display name — and a
malformed user template still lists, by folder name with the loader's
own sentence on the row, never failing its neighbours. The store is
re-scanned on every presentation and on app activation, the Reveal
round trip made honest without watching a folder 09 deliberately
leaves unwatched; Reveal lives in the chooser's header and mints the
store on first press. Save as Template repeats Duplicate's sequence —
progress row with Cancel, flush, detached cancellable copy — through
the engine: mint the store, read the next user order before the copy
can count itself, Finder-ladder the name, copy excluding .git and
.trash/, then stamp the whole template: mapping on the landed copy
through updateIndex, with no bracket because the copy lives outside
every watched board. Folder attributes deliberately don't carry — the
one lock the command stays live under is the read-only-DMG one, and
carrying its mode bits would mint a read-only template in the user's
own store; the command gates instead on the real hazard, unsaved card
content. A signpost names the template only when the ladder renamed
it. One name ladder now serves Duplicate and the store.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/Storage/BoardLoader.swift

@@ -82,7 +82,12 @@ public enum BoardLoader: Sendable {
     /// Board-level key for `BoardModel.template` — not schema-owned in the engine's sense
     /// (`FrontmatterKeys.schemaOwned`), because its value is opaque and read raw here rather
     /// than through a typed `FrontmatterDocument` accessor.
-    private static let templateKey = "template"
+    ///
+    /// Internal rather than `private` for `indexFileName`'s reason: `TemplateEngine` writes this
+    /// key on a Save as Template copy — "the one writer of keyed files is Save as Template"
+    /// (09-templates.md ▸ Storage) — and the reader and that one writer must never disagree about
+    /// how it is spelled.
+    static let templateKey = "template"
 
     /// Internal rather than `private`: `BoardWriter` names the same file, and the loader and
     /// the writer must never disagree about which file a folder's content lives in.