fix(tui): prevent session picker crash

[opencode-agent]

Closes#36747
Closes#36773
Supersedes #36754

Cause

#36711 made dialog action availability depend on the selected option. Footer focus still translated each rendered action back through actionItems().indexOf(item), re-entering that selection-dependent memo while the session picker registered its actions and exposing an uncomputed value.

Fix

• Track focused footer actions by stable command name instead of positional index.
• Initialize the option/selection graph before registering action bindings that can evaluate selection-dependent availability.
• Clear focus when the command becomes unavailable rather than transferring it to a different action at the same index.
• Add regression coverage for a focused action becoming disabled.

Verification

• bun test --timeout 30000 --only-failures from packages/tui (220 pass, 1 skip)
• bun typecheck from packages/tui
• bun x prettier --check packages/tui/src/ui/dialog-select.tsx packages/tui/test/cli/tui/dialog-select.test.tsx
• OpenCode Drive: while a model response is streaming, /sessions opens the Sessions dialog without hitting the error boundary

[github-actions]

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