Orphaned .backgrounds/ files get tidied — a repoint trims the one it leaves behind, and every open sweeps what got away

Follow-up to d0c5461's .backgrounds/ folder: generating, pasting, or
choosing solid now trims the app's own prior file in .backgrounds/ as
part of the same write when it repoints or unsets background.image
away from it — silent, best-effort, never blocking the gesture that
triggered it. A scheduled heal at every board open sweeps whatever
that trim declined or missed: any .backgrounds/ file the board's
current background.image no longer names, announced with a loss-row
notice in the loose-file relocation's own voice. Legacy root-level
references are untouched by both paths — the tidy scopes to
.backgrounds/ only, since that is the one folder the app can prove it
wrote into.

Removal is via FileManager.trashItem, matching the attachment-removal
precedent (recoverable, never a hard delete).

Supersedes applySolidBackground's earlier "facets.png survives on
disk" contract for the settled case: the ruling reads "unsets" as one
more shape of "repoints away from a .backgrounds/ file", so a settled
solid choice now trims the generated picture it displaces, same as
switching producers does. The superseded test and doc comment are
updated to the new behavior; the echo-window (no-reload-yet) case is
unaffected and still leaves the file in place.

Claude-Session: https://claude.ai/code/session_014PtZdPwqZuqEDLc6wZMtEy

Kanban/LiveStore/BannerCenter.swift

@@ -602,6 +602,27 @@ public final class BannerCenter {
         postLoss(message)
     }
 
+    /// **The orphaned background-image sweep** (01-storage-format.md § Validation and healing, ruled
+    /// 2026-08-09: "the open-time heal posts the calm notice-family banner the loose-file relocation
+    /// heal uses — state what was tidied"): a file in `.backgrounds/` that nothing on the board named
+    /// any more was moved to the system Trash, and this is the row that says so.
+    ///
+    /// **A loss row, on `postRelocatedLooseFiles`' exact reasoning**: the app removed a file on its own
+    /// initiative, on a board it opened rather than on a gesture the user made — that must be said out
+    /// loud, must not evaporate unread, and must not rank as an error, because nothing failed.
+    ///
+    /// **Only the sweep posts this.** The in-flow replace-in-place trim a background apply makes on
+    /// its own prior file is deliberately silent — the ruling's "temp-file posture": the app
+    /// superseding its own file inside a gesture the user already sees succeed (a fresh picture lands)
+    /// is not news the way a heal discovered on open is.
+    ///
+    /// `names` are the bare filenames the sweep actually removed, in the order it removed them. A
+    /// sweep that removed nothing posts nothing.
+    public func postTidiedBackgroundImages(_ names: [String]) {
+        guard let message = Self.tidiedBackgroundImagesMessage(for: names) else { return }
+        postLoss(message)
+    }
+
     /// **The skip notice** (01-storage-format.md § Malformed input, ruled 2026-07-31): the decision
     /// surface offered Skip on a defect the app has no honest repair for, the user consented, the
     /// board opened without that item — "the file stays on disk untouched, tolerated-invisible like
@@ -973,6 +994,15 @@ public final class BannerCenter {
             // failure the user did not provoke is exactly the one they have no other way to learn
             // about.
             "Couldn't move '\(filename)' into attachments"
+        case let .tidyBackgroundImage(filename):
+            // **The successful sweep's own verb, negated** — `postTidiedBackgroundImages`' "Removed
+            // '\(filename)' …" said the other way round, `.relocateLooseFile`'s own precedent: the
+            // failure reads as the same sentence's negation rather than a different event. It stays
+            // in the ordinary one-shot precedence class for `.relocateLooseFile`'s reason too — this
+            // is work the app started on its own, and a failure the user did not provoke is exactly
+            // the one they have no other way to learn about. In practice only the sweep's own failure
+            // ever reaches here; the in-flow trim swallows its own (the ruling's "temp-file posture").
+            "Couldn't remove '\(filename)'"
         case .agentGuide:
             // **"the agent guide", not "CLAUDE.md"**: the file is the app's, written for agents, and
             // most users will never have opened it — a filename here would name something they have
@@ -1284,6 +1314,24 @@ public final class BannerCenter {
         return "Repaired duplicate id — \(sole(only))"
     }
 
+    /// The orphaned background-image sweep's line, `migratedTombstonesMessage`'s voice: the act
+    /// first, the subject after an em dash, plurals folded, a tail naming the cause — without it the
+    /// sentence would read as something that happened *to* the board rather than the app's own
+    /// housekeeping.
+    ///
+    /// - **One**: "Removed 'facets 2.png' — it was no longer the board's background".
+    /// - **Several**: "Removed 3 background images — they were no longer the board's background".
+    ///
+    /// `nil` when nothing was removed — a sweep that swept nothing is not news.
+    public nonisolated static func tidiedBackgroundImagesMessage(for names: [String]) -> String? {
+        guard !names.isEmpty else { return nil }
+        let subject = names.count == 1 ? "'\(names[0])'" : "\(names.count) background images"
+        let tail = names.count == 1
+            ? "it was no longer the board's background"
+            : "they were no longer the board's background"
+        return "Removed \(subject) — \(tail)"
+    }
+
     /// The lossy export's line, in the relocation family's voice — the act first, the cause after an
     /// em dash, plurals folded into their counts.
     ///