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).

test/config.test.ts

@@ -595,6 +595,34 @@ describe('loadConfig — v0.7 B1 diff + B2 statusline', () => {
   })
 })
 
+// ── W3 quick-wins (b) cost budget guard ───────────────────────────────────────
+describe('loadConfig — W3 COST_BUDGET_USD', () => {
+  beforeEach(() => {
+    mockNetworkInterfaces.mockReturnValue({})
+    mockHomedir.mockReturnValue('/home/testuser')
+  })
+
+  it('defaults costBudgetUsd to 0 (disabled) when unset', () => {
+    expect(loadConfig({}).costBudgetUsd).toBe(0)
+  })
+
+  it('parses a float dollar value', () => {
+    expect(loadConfig({ COST_BUDGET_USD: '5.50' }).costBudgetUsd).toBe(5.5)
+  })
+
+  it('accepts an explicit 0 (disabled)', () => {
+    expect(loadConfig({ COST_BUDGET_USD: '0' }).costBudgetUsd).toBe(0)
+  })
+
+  it('throws for a non-numeric value', () => {
+    expect(() => loadConfig({ COST_BUDGET_USD: 'abc' })).toThrow(/COST_BUDGET_USD/)
+  })
+
+  it('throws for a negative value', () => {
+    expect(() => loadConfig({ COST_BUDGET_USD: '-1' })).toThrow(/COST_BUDGET_USD/)
+  })
+})
+
 describe('loadConfig — v0.7 B3 worktree', () => {
   beforeEach(() => {
     mockNetworkInterfaces.mockReturnValue({})