Give card windows their own undo stacks and coarsen the close

Phase B of the two-level undo card: every card-window gesture — comment
post/delete/edit, body Edit sessions, style and details changes —
registers fine-grained on the window's own stack (window.undoManager
answers with it; board ⌘Z never sees mid-session card steps; an empty
window stack beeps, never falls through). Window close folds the stack
into one coarse values-based board step ("Edit card 'X'") — per-target
per-field later-wins merge, so foreign mid-session writes stay out by
construction, a no-net-change session registers nothing, and any stale
component skips the whole step. The comments/.trash purge defers with
the coarse step via a step-retirement seam on the providers: it runs
when the step leaves the board stack or the board session ends; the git
provider retires dropped steps on register, which keeps Pro's
purge-at-close-flush structural with no tier check. Interim on git
boards: gestures still auto-commit per debounce until phase C's
close-flush commit.

2432 tests in 418 suites green.

Claude-Session: https://claude.ai/code/session_01CqjXB7ASoWtbyoGod68k97

Kanban/Storage/AgentGuide.swift

@@ -68,8 +68,16 @@ enum AgentGuide {
     /// Trash, re-ruled 2026-07-31; 08-agent-integration.md's own line): the trash sorts by `modified`
     /// descending, so there is no rank to mint on the way in — the guide's smallest-`order`-minus-1024
     /// formula is replaced by "restamp `modified`, leave `order` alone", which is the same stamp
-    /// discipline v7 already taught, now doing the ordering as well.
-    static let version = 8
+    /// discipline v7 already taught, now doing the ordering as well. **v9 teaches the one-line
+    /// value**, from a real agent incident (2026-07-31): a card's `title` was a double-quoted
+    /// scalar wrapped across two lines, and a hand copy of that card onto another board took the
+    /// first line without its continuation. An unclosed quote does not stop at the key it began
+    /// on — it runs to the end of the block — so a board failed to load over a file whose only
+    /// defect was a missing second line, and the parser's complaint pointed at the *last* key it
+    /// swallowed rather than the title. The guide now teaches long values as one long line, says
+    /// why the wrapped shape is the dangerous one to copy, and names the unterminated quote beside
+    /// the unquoted colon in Hard rules.
+    static let version = 9
 
     // MARK: - The version marker
 
@@ -381,6 +389,16 @@ enum AgentGuide {
     containing `: ` or starting with `#`, `[`, `{`, or a quote — when in doubt,
     double-quote.
 
+    **Keep every value on one line.** A double-quoted scalar may legally
+    continue on the following line (`title: "Long title` then `  the rest"`),
+    and some tools emit that shape for long titles — but it is the most common
+    way frontmatter gets broken by hand: the continuation reads as a line of
+    its own, so an edit or a copy that takes only the first one leaves the
+    quote unclosed, and an unclosed quote runs on to swallow every key below
+    it. The whole file then fails to parse, not just the title. Titles have no
+    length limit — write a long one as one long line, and when you copy a card
+    between boards, copy its frontmatter block whole.
+
     Unknown keys are preserved verbatim by the app and invisible in its UI —
     custom metadata (`project:`, `tags:`, `claimed-by:` …) is safe to add and
     survives every app rewrite. Reserved for Lanework's upcoming tracker sync —
@@ -491,8 +509,9 @@ enum AgentGuide {
     ## Hard rules (the app fails loudly on violations)
 
     - Frontmatter must parse as YAML; `schema` (plus `order` on lanes and
-      cards) is required. Keep `schema: 1`. The classic violation is an
-      unquoted colon in a title (see Frontmatter above).
+      cards) is required. Keep `schema: 1`. The classic violations are an
+      unquoted colon in a title and a quoted value left unclosed across a
+      line break (see Frontmatter above).
     - Files must be UTF-8 without BOM.
     - Never create a card folder without an `index.md`.
     - Never rename UUID folders.