fix(tui): align session tab shortcut labels

[kitlangton]

What

Treat session tab markers as keyboard shortcut labels rather than ordinal numbers:

• tabs 1-9 render their matching digit
• tab 10 renders 0 and is selectable with <leader>0 or ctrl+0
• later tabs render a subdued ·
• every marker uses the same two-cell gutter, so titles and vertical-tab details stay aligned

Before / After

Before

Tabs rendered their raw one-based position. Tabs 10 and later widened the gutter, shifted their title/detail alignment, and displayed numbers that did not correspond to direct tab shortcuts.

9  Ninth tab
10 Tenth tab
11 Eleventh tab

After

The first ten markers correspond exactly to direct shortcuts. Unbound tabs use a neutral marker without implying a multi-digit key sequence.

9  Ninth tab
0  Tenth tab
·  Eleventh tab

How

• session-tabs-model.ts defines the shared zero-based marker mapping.
• session-tabs.tsx uses the mapping and a fixed gutter in both vertical and horizontal layouts.
• app.tsx and the v1 keybind bridge expose session.tab.select.10 with <leader>0,ctrl+0.

Scope

This does not add direct shortcuts beyond the tenth tab or change the separate pinned-session quick slots, which remain limited to nine.

Testing

• bun run test in packages/tui: 608 passed, 5 skipped
• bun run typecheck in packages/tui
• bun run typecheck at the workspace root via the push hook: 33 packages passed
• bunx prettier --check on changed files
• OpenTUI capture of the 12-tab fixture in vertical orientation

Demo

The existing twelve-tab fixture shows 9, 0, then two aligned middle dots. The selected twelfth tab retains the normal active treatment.