feat(mecatui): display effective token budget in the UI

Problem

Mecatui does not show the effective per-run token budget. Once embedded-server configuration can set a deployment ceiling and a Subagent call can further tighten it, an operator or user cannot tell which limit applies, whether the budget is unlimited, or how much of it has been consumed.

Desired change

Display the effective token-budget state in mecatui for the active main run and delegated runs where that information is available. The UI must distinguish an unlimited budget from a configured ceiling and make the unit explicit.

Acceptance criteria

• The main-session UI shows either unlimited or the effective per-run token ceiling and current usage.
• Delegation surfaces (at least Subagent; Team/Parallel where applicable) show the effective child budget or clearly state when no separate budget applies.
• If both an operator/deployment ceiling and a per-call child max_run_tokens exist, display the effective tighten-only minimum rather than a misleading requested value.
• A budget stop (stop=budget) is visually connected to the displayed budget state.
• The design accommodates the proposed weighted-cost budget work without presenting an ambiguous “token” label if enforcement changes units.
• The data comes from an authoritative server-side projection; the TUI must not independently recompute budget semantics from streamed token deltas.
• Add focused UI/client tests and update user-facing documentation.

Context

Mecatui renders relayed proto events and should remain a client rather than import engine accounting. Existing server-side budget semantics are in engine/agent/loop.go (effectiveMaxRunTokens, budgetExhausted), while the footer already receives server-resolved model/context information. The embedded server currently defaults MaxRunTokens to unlimited; see #686.