T20: SDK remote budget config (pull from dashboard)

[bmdhodl]

Context

Close the loop: users configure budgets in dashboard (T10), SDK picks them up automatically.

Acceptance Criteria

• BudgetGuard.from_remote(api_key="ag_...", budget_name="production") class method
• Fetches config from GET /api/v1/budgets?name=production using urllib (zero deps)
• Falls back to provided local defaults if network fails
• Caches config for 5 minutes (avoid per-call HTTP)
• Thread-safe (multiple agents sharing one guard)
• Test with mock server

Key Files

• sdk/agentguard/guards.py — BudgetGuard class

Week: 10-11 | Depends on: T10

[bmdhodl]

Blocked by: #28

Quality gate: tests pass, lint clean, security checklist done.