feat: persist the reconcile cache to disk — instant boot + offline reads

Extends the in-memory cache into a durable mirror. The reconcile snapshot is
written to disk on every successful reconcile; on boot the app shows it instantly
(stale-while-revalidate) instead of a blank board, and if gitea is unreachable,
reads fall back to it (offline). Rebuildable — the durable truth stays in gitea.

- snapshot-store.ts: load/save the snapshot as JSON in app userData (never throws;
  corrupt/absent → "no cache"). At this scale (~34 issues, 37KB) the whole snapshot
  fits in memory, so a JSON file beats indexed SQL — no query benefit yet, no
  native-module (better-sqlite3/electron-rebuild) or WASM dependency. That's the
  next step if the mirror ever needs indexed queries over larger data.
- gitea.ts: getSnapshot persists on a fresh pull; bootSnapshot() returns the
  persisted snapshot (without seeding the cache — agents still reconcile fresh);
  gitea:boot serves it; gitea:reconcile falls back to it on failure (stale:true).
- useBacklog: stale-while-revalidate — boot instantly, then a fresh reconcile
  supersedes; a reconcile error keeps the shown snapshot instead of erroring.

Verified: desktop typecheck clean, 14 fixture e2e green. Live: the snapshot
persists (34 issues / 44 deps / 34 timelines / 5 milestones written to disk); a
second launch with gitea unreachable renders the full real board — NOW/NEXT/LATER
+ the Monte Carlo cone — entirely from the cache (new live-persistence e2e).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/desktop/src/main/gitea.ts

@@ -27,6 +27,8 @@ import {
 } from '@commitea/core'
 import { ipcMain } from 'electron'
 
+import { loadSnapshot, saveSnapshot } from './snapshot-store.js'
+
 /** Walk up from cwd looking for a .env.local with a GITEA_TOKEN (dev convenience). */
 function loadEnvLocalToken(): string | undefined {
   let dir = process.cwd()
@@ -145,6 +147,7 @@ export async function getSnapshot(client: GiteaClient, opts?: { maxAgeMs?: numbe
   }
   const snap = await reconcileSnapshot(client)
   snapshotCache = { snap, at: Date.now() }
+  saveSnapshot(snap, new Date().toISOString()) // persist for instant boot + offline
   return snap
 }
 
@@ -153,6 +156,25 @@ export function invalidateSnapshot(): void {
   snapshotCache = null
 }
 
+/**
+ * The last persisted snapshot (from a previous session), for instant boot. The
+ * renderer shows it immediately, then a real reconcile supersedes it
+ * (stale-while-revalidate). Returns null when there's nothing on disk; its
+ * `savedAt` marks staleness. It does NOT seed the cache — agent tool calls
+ * always reconcile fresh so they never reason over stale data.
+ */
+export function bootSnapshot(): (Snapshot & { savedAt: string }) | null {
+  const persisted = loadSnapshot()
+  if (!persisted) return null
+  return {
+    issues: persisted.issues,
+    milestones: persisted.milestones,
+    deps: persisted.deps,
+    timelines: persisted.timelines,
+    savedAt: persisted.savedAt,
+  } as unknown as Snapshot & { savedAt: string }
+}
+
 /** Agent tool calls tolerate a slightly stale snapshot (seconds) to stay responsive. */
 export const AGENT_SNAPSHOT_TTL_MS = 30_000
 
@@ -162,11 +184,25 @@ export function registerGiteaIpc(): void {
 
   ipcMain.handle('gitea:status', () => ({ configured: !!client, repo }))
 
+  // Instant boot: the last persisted snapshot, shown before the fresh reconcile lands.
+  ipcMain.handle('gitea:boot', () => {
+    if (!client) return { configured: false }
+    const persisted = bootSnapshot()
+    return persisted ? { configured: true, cached: true, ...persisted } : { configured: true, cached: false }
+  })
+
   ipcMain.handle('gitea:reconcile', async () => {
     if (!client) return { configured: false, issues: [], milestones: [], deps: [], timelines: {} }
-    // explicit UI sync — force fresh, and warm the cache for agent tool calls
-    const snap = await getSnapshot(client, { maxAgeMs: 0 })
-    return { configured: true, ...snap }
+    try {
+      // explicit UI sync — force fresh, and warm the cache for agent tool calls
+      const snap = await getSnapshot(client, { maxAgeMs: 0 })
+      return { configured: true, stale: false, ...snap }
+    } catch (e) {
+      // offline / gitea down — serve the last persisted snapshot rather than error out
+      const persisted = bootSnapshot()
+      if (persisted) return { configured: true, stale: true, ...persisted }
+      throw e
+    }
   })
 
   ipcMain.handle('gitea:getIssue', async (_event, index: number) => {