mecatui: migrate agent-definition inventory to bounded.Viewport #1897

Scope

Migrate the read-only /agents inventory in cmd/mecatui/ui/agents_inventory.go to bounded.Viewport.

Current behavior

The surface owns an explicit physical-line scroll offset over ANSI-safe wrapped lines and deliberately uses a fixed 14-line card budget. It has no selection or activation.

Required outcome

• Centralize physical offset and geometry in bounded.Viewport.
• Retain the fixed 14-line budget, existing ANSI-safe row construction, keyboard behavior, and reset-on-RPC-result behavior.
• Keep headers, overflow wording, and interaction surface-owned.

Verification

Add scenario-first acceptance coverage and run focused agents-inventory/UI and bounded tests before project gates.