Build the template engine — board-as-template instantiation

A template is a board folder the ordinary loader reads — no second
schema, no Swift catalog. BoardTemplate became exactly that: a loaded
BoardModel with chooser-facing derivations, the lane-title stub gone.
TemplateEngine instantiates by the copy-remint-restamp walk: .git and
.trash excluded at top level only — both names mean something at a
board root and nowhere else, and .gitignore must survive — every
materialized folder reminted, created/modified stamped fresh (born
today, not forked), modified-by cleared, the template: key carried
inert, the blurb and style inherited, and loose card files normalized
at this import boundary per the paste precedent so a new board never
opens with a notice about a mess its own birth made. Legacy deleted:
keys copy through verbatim to the one migrator — stripping would
resurrect, skipping would destroy. Atomicity is construct-then-clean:
a sibling temp can be sandbox-refused and a cross-volume rename is
just a second copy, so the call removes what it created on every
non-board exit and never touches an occupied destination. The
cancellable per-item walk extracted into BoardTreeCopy serves
Duplicate and instantiation with two parameters — top-level exclusions
and folder-attribute carriage, the only axes they differ on.

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

README.md

@@ -30,7 +30,7 @@ Lanework is in early development. This list tracks what has actually shipped and
 
 - **The welcome screen** — branding and two actions on the left, recents on the right: board icon, name, containing folder, and the lane/card counts stamped at last close, newest first. The list never opens a board to build itself, so a huge board or an offline volume costs nothing. Single click selects, double click or Return opens, and a context menu carries Open, Reveal in Finder, and Forget. A board that failed to open or restore says so **on its own row**, in the warning tint, carrying the loader's specifics rather than a modal at launch; a board whose bookmark no longer resolves dims to Unavailable with Open and Reveal off and Forget still live; and a failure naming no known board keeps a list of its own rather than vanishing. File ▸ Open Recent lists the same boards — unavailable ones disabled — with Clear Menu at the bottom, which forgets every record because here the registry *is* the menu.
 
-- **New Board and Duplicate** — File ▸ New Board… (⌥⌘N) opens a Pages-style template chooser with a mini per-lane preview per template, then a save panel for the location; the new board's frontmatter title is the document name you chose, so the window title and the Finder name start out matching. File ▸ Duplicate (⇧⌘S) forks the frontmost board to a Finder-style "Board copy" sibling (then "copy 2", "copy 3"), preceded by the pending-work flush so the copy misses nothing and with the original left open beside it. The copy is literal: every GUID kept — a whole-board copy is a new identity namespace — `.trash/` carried along so the copy matches its own copied history, strays and timestamps untouched.
+- **New Board and Duplicate** — File ▸ New Board… (⌥⌘N) opens a Pages-style template chooser with a mini per-lane preview per template, then a save panel seeded with the template's own name; the new board's frontmatter title is the document name you chose, so the window title and the Finder name start out matching. **A template is itself a board** — an ordinary schema-valid folder read by the ordinary loader, so its display name, icon, blurb and lanes all come off its own `index.md`, and authoring one is adding a folder rather than writing code. Creating from one copies that folder: fresh GUIDs for every lane and card, `created`/`modified` stamped today (born, not forked from the template), the blurb becoming the new board's description, attachments and card bodies byte for byte, strays and symlinks carried verbatim — and `.git` and `.trash/` deliberately left behind, so a new board is never silently in git mode and never born with trash. Nothing half-made is ever left where you pointed: a create that fails or is cancelled removes its own partial, and a name already taken is refused rather than replaced. File ▸ Duplicate (⇧⌘S) forks the frontmost board to a Finder-style "Board copy" sibling (then "copy 2", "copy 3"), preceded by the pending-work flush so the copy misses nothing and with the original left open beside it. The copy is literal: every GUID kept — a whole-board copy is a new identity namespace — `.trash/` carried along so the copy matches its own copied history, strays and timestamps untouched.
 
 - **The card window** — ⌘↩ or a double-click opens a card in its own window: two full-height, independently scrolling columns — a wide body column and a narrow attributes sidebar whose fixed width is derived from font metrics, so the window's resize flex all goes to the body. The title bar carries the card's title live and subtitles it "⟨board⟩ › ⟨lane⟩", following the card as it moves between lanes and re-reading the board's name as it's renamed. The body column shows the title, a quiet created/modified/by line built from whichever frontmatter keys exist, and the body itself; the sidebar leads with the Attachments section (below) and its remaining sections are stacked headers awaiting their content. Reopening a card focuses the window it already has, and the window closes itself the moment its card stops being on the board — moved to the trash (entering the trash counts as deleted), gone with its deleted lane, purged, or moved to another board; a dirty Edit buffer flushes into the card's new location first, so the keystrokes survive a later restore.