feat: enforce global budget cap across all agents

[kaissb]

What changed

• internal/budget/manager.go — BudgetManager now tracks a global running total (globalState.spentUSD). CheckBudgetDetailed checks the global limit before per-agent limits; if limit_usd > 0 and spentUSD + estimatedCost > limit → reject with code global_budget_exceeded.
• Period-based auto-reset: maybeResetGlobalPeriodLocked resets the global counter when the current period expires (hourly / daily / weekly / monthly).
• GetGlobalBudget() exposes a GlobalBudgetView snapshot consumed by the API.
• internal/api/server.go — GET /budgets now uses GetGlobalBudget() and returns the enriched global object (period_starts_at, period_resets_at, percentage_used). Removed redundant globalPeriod/globalLimitUSD fields from Server.
• dashboard/svelte — Overview page fetches /budgets and renders a Global Budget KPI card (spend bar + % used) when limit_usd > 0.
• scripts/install.sh — fixed to handle .tar.gz release archives.
• Dockerfile — added WORKDIR /data.

Tests

• 6 new unit tests in internal/budget/manager_test.go: not-configured → no enforcement, under limit → allow, exceeded → rejects all agents, global checked before per-agent, period reset, GetGlobalBudget view.
• All packages pass: go test -race ./... (83.4% total coverage).
• golangci-lint: 0 issues. go vet: clean. Dashboard build + svelte-check + tsc: clean.
• Integration test: scripts/dev/test-global-budget.sh