feat(cockpit): quick wins — sync chip, cost budget guard, digest, recent commits (W3)

Four small, high-delight features that turn passive capture into glanceable signals.

- Sync chip on project cards: ahead/behind vs upstream + last-commit time, folded
  into the existing concurrent per-repo metadata pass (git rev-list --count
  --left-right @{u}...HEAD + git log -1 --format=%ct; no upstream → undefined, no route).
- Cost budget guard: COST_BUDGET_USD env (0 = off); a per-session one-shot latch
  (Session.budgetNotified, cost is monotonic so never re-armed) fires a single push
  on threshold crossing in manager.handleStatusLine; the already-broadcast telemetry
  frame carries the warn (tg-cost-warn styling derived from costUsd>=budget via
  /config/ui — no new ServerMessage). web-push title added to sw-push.js.
- "While you were away" digest: GET /digest?since= → {finished, needsInput, stuck,
  totalCostUsd, sessions[]} aggregate over manager.list(); FE banner on reconnect.
- Recent commits per project: src/http/git-log.ts (NUL-delimited git log → CommitInfo[]),
  GET /projects/log?path= (isValidGitDir), textContent-inert render in project detail.

All git via execFile (no shell) + validated cwd; new routes read-only; commit
messages rendered via textContent. Verified: typecheck + build:web clean, 1904 pass
at --test-timeout=30000 (two default-5s failures are slow-sandbox real-subprocess
timeout flakes — the known ring-buffer test + a new real-git-clone sync test — not
logic regressions).

public/main.ts

@@ -22,6 +22,7 @@ import { mountShortcuts } from './shortcuts.js'
 import { mountShareSession } from './share.js'
 import { mountGridToggle, matchFocusCycleKey } from './grid-layout.js'
 import { mountGridPresets } from './grid-presets.js'
+import { mountDigest } from './digest.js'
 
 const paneHost = document.getElementById('term')
 const tabs = document.getElementById('tabs')
@@ -118,6 +119,11 @@ mountGridPresets(toolbar, {
 
 mountQrConnect(toolbar)
 
+// W3(c): "while you were away" reconnect digest — one compact dismissible banner
+// summarising what finished / needs input / got stuck since this device's last
+// visit. Best-effort (no banner on fetch failure); advances its own last-seen.
+void mountDigest(document.body)
+
 // PWA: register the service worker (installable + offline shell, M4).
 if ('serviceWorker' in navigator) {
   window.addEventListener('load', () => {