Introduces stable per-tab IDs and a global `--tab <id>` flag for scoping individual commands to a specific tab.
Breaking change: response payloads for `tab_list`, `tab_new`, `tab_switch`, `tab_close`, and `window_new` now use `tabId` instead of `index`. `tab_close` returns `{tabId, closed: true}` instead of `{closed, activeIndex}`. `agent-browser tab <unknown>` now errors instead of silently listing tabs.
Follow-up PR to land immediately after this fixes a compile error on the provider direct-page path, clears per-tab daemon state around scoped switches, and implements active-tab restoration so `--tab N` is non-intrusive as intended.
