An unopenable repository fails loudly — the standing row, the paused surface, the honest heal

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

Kanban/Git/GitRepository.swift

@@ -237,6 +237,29 @@ enum GitRepository {
 
     // MARK: Reads
 
+    /// **Whether libgit2 can open the repository at the board root at all** — the detection-time
+    /// probe behind 06-history-undo.md ▸ Rules' corrupt-`.git` loud failure (ruled 2026-07-31):
+    /// "a corrupt or unopenable repo never falls to mode none … the failure is **loud**".
+    ///
+    /// It is deliberately the *same* call every read here already makes (`Repository.open`), so
+    /// "unreadable" means exactly what it means to the rest of this file rather than being a second
+    /// opinion about the same repository. `git_repository_open` validates the layout — `HEAD`,
+    /// `objects/`, `refs/` — resolves a `gitdir:` pointer file, and refuses a repository whose
+    /// format version or extensions it does not implement, which is why a SHA-256 repository lands
+    /// here "by construction" (06 ▸ Repository hygiene: "an adopted SHA-256 repo the engine cannot
+    /// open takes the corrupt-repo loud-failure path").
+    ///
+    /// A board with no `.git` at all answers `false` too — there is no repository to read — but that
+    /// is not a state any caller reaches: the probe runs only in mode `git`, which is exactly the
+    /// mode a root `.git` defines.
+    ///
+    /// Read-only, like everything in this section: opening a repository writes nothing, and a
+    /// repository that fails to open has not been touched at all.
+    nonisolated static func canOpen(at boardRoot: URL) -> Bool {
+        guard BoardGitMode.hasGitEntry(at: boardRoot) else { return false }
+        return (try? Repository.open(at: boardRoot)) != nil
+    }
+
     /// The current branch's short name, or `nil` when there is no repository at `boardRoot` or
     /// libgit2 cannot open it — the popover's read-only branch line (03-board-ui.md ▸ Board
     /// popover), and nothing more: branch switching and creation are a later card.