Build AgentGuide — versioned CLAUDE.md maintenance

The app-owned agent guide at every board root (DESIGN/08 ▸ The agent
guide): version-gated by a first-line marker (v5, superseding the
pathfinder's v4 guides on real boards), rewritten when missing or older,
byte-for-byte untouched when current or newer. A markerless CLAUDE.md is
displaced to CLAUDE.user.md when that name is free — never clobbered —
and the guide write is skipped with a log when it isn't. Symlinks,
folders, and read-only volumes are skipped in silence; the write rides
performWrite's bracket as an app-mediated Writer operation (new
WriteOperation.agentGuide), so the echo lands appMediated and the
Pro-era committer can attribute it honestly later.

Hooked at store acquire (beside the loose-file relocation, after the
watcher exists) and on every successful reload — the guide self-heals
from foreign deletion or rollback, pre-wiring 06's acknowledged undo
bounce. The refresh memo arms before each attempt and clears on a
successful write, so a failing write can't hot-loop and a foreign
deletion stays healable. First-line-only marker parsing (no Regex);
guide content is one swappable literal, finalized under the next card.

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

Kanban/LiveStore/BannerCenter.swift

@@ -697,6 +697,13 @@ 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 .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
+            // no relationship with. It says nothing about the board's own files because none were
+            // touched, and nothing is lost: the board works exactly as well without the guide, which
+            // is why every *refusal* to write it is a log line and only a real I/O failure gets here.
+            "Couldn't update the agent guide"
         case let .toggleTask(title):
             // The user's word for it, not the file's: they ticked a box. The card is named where
             // the read that preceded the flip learned its title, so a body write that refused says