fix(electrobun): complete all 16 Codex #3 findings

CRITICAL:
- Message persistence race: snapshot batchEnd before async save
- Double-start guard: startingProjects Set prevents concurrent launches
- Symlink path traversal: fs.realpathSync() in path-guard.ts
- Relay false success: connect() returns { ok, machineId, error }

HIGH:
- Session restore skips if active session exists
- Remote remove: new RPC, cleans backend map
- Task board poll token: stale responses discarded after drag-drop
- Health concurrent tools: toolsInFlight counter (was boolean)
- bttask transactions: delete wraps comments+task, addComment validates
- PTY buffer cleared on reconnect
- PTY large paste: chunked String.fromCharCode (8KB chunks)
- Sidecar max line: 10MB limit prevents unbounded memory
- btmsg authorization: group validation, channel membership checks

MEDIUM:
- Session retention: max 5 per project, purgeSession/untrackProject
- Relay IPv6: URL parser replaces string split
- PTY schema: fixed misleading base64 comment

ui-electrobun/src/mainview/settings/RemoteMachinesSettings.svelte

@@ -84,10 +84,11 @@
     } catch { /* ignore */ }
   }
 
+  // Fix #6 (Codex audit): Use remote.remove RPC that disconnects AND deletes
   async function handleRemove(machineId: string) {
     try {
-      await appRpc.request['remote.disconnect']({ machineId });
-    } catch { /* may already be disconnected */ }
+      await appRpc.request['remote.remove']({ machineId });
+    } catch { /* may already be disconnected/removed */ }
     removeMachine(machineId);
   }
 
@@ -131,6 +132,9 @@
             <span class="status-text" style:color={statusColor(m.status)}>
               {statusLabel(m.status)}
             </span>
+            {#if m.error}
+              <span class="machine-error" title={m.error}>{m.error}</span>
+            {/if}
           </div>
           <div class="machine-actions">
             {#if m.status === 'connected'}
@@ -239,6 +243,7 @@
   }
 
   .status-text { font-size: 0.6875rem; font-weight: 500; }
+  .machine-error { font-size: 0.625rem; color: var(--ctp-red); max-width: 10rem; overflow: hidden; text-overflow: ellipsis; white-space: nowrap; }
 
   .machine-actions { display: flex; gap: 0.25rem; flex-shrink: 0; }