fix(tui): select first row when opening terminals tab

[kitlangton]

Why

Opening the Terminals tab with no visible terminal leaves every row unselected. When the list is empty, even the only action, + New terminal, requires an arrow key before Enter does anything.

What Changes

Fall back to the first row when there is no matching visible terminal.

State on opening	Selection
No terminals	+ New terminal
Existing terminals, none visible	First terminal
A listed terminal is visible	That terminal

Demo

terminal-focus-before-after.mp4

Matched OpenCode Drive runs of the production TUI: before cd504dc66a, after bc784ff718. Both use the same isolated project, simulated model response, 90×24 terminal viewport, and interaction: open the picker, switch to Terminals, then press Enter. The clips play sequentially at native resolution for legibility, with equal-duration holds and no acceleration. Before, Enter leaves the picker open; after, it opens the terminal pane.

Scope

One-line change to the Terminals tab's initial-selection fallback.

Verification

# packages/tui
bun typecheck

# Same script, two development checkouts
opencode-drive check terminal-focus-demo.ts
opencode-drive start --name terminal-focus-before --record --script terminal-focus-demo.ts --dev ../opencode-terminal-tab-before
opencode-drive start --name terminal-focus-after --record --script terminal-focus-demo.ts --dev ../opencode-terminal-tab-focus

git diff --check

All passed. Both recorded runs completed and their frames were inspected. The pre-push hook also passed all 33 typecheck tasks using Bun 1.4.2 (27 cached).