feat(tui): add vertical session tabs

[kitlangton]

What

Adds an optional vertical session-tab rail to the V2 full-screen TUI.

• Enable it from /settings → Tabs → Vertical.
• Shows numbered, two-row tabs with the project name beneath the title.
• Uses a responsive 20–30 column rail with scrolling and active-tab tracking.
• Preserves the existing horizontal tab strip when vertical tabs are disabled.
• Carries running, completion, warning, and error states through subtle two-row pulses and split-cell separator gradients.

How

• packages/tui/src/app.tsx selects horizontal or vertical placement.
• packages/tui/src/component/session-tabs.tsx renders the rail, metadata, scrolling, drag reordering, title truncation, and status colors.
• packages/tui/src/component/tab-pulse.tsx supports independent inner/outer animation channels so adjacent tabs blend without coupling their transitions.
• packages/tui/src/context/session-tabs-model.ts centralizes responsive rail sizing.
• packages/tui/src/routes/session/index.tsx accounts for the occupied rail width in session layout calculations.
• packages/tui/src/config/index.tsx and dialog-config.tsx expose tabs.vertical.
• bun run dev:live connects a worktree TUI to the elected opencode2 service while reusing the next TUI state channel.

Scope

• This changes only the V2 full-screen TUI. Desktop and web layouts are unchanged.
• Session activity timestamps are intentionally not shown because assistant/tool activity does not currently advance session.time.updated; tracked in V2: session.time_updated goes stale during active turns, breaking recency sort #36893.

Testing

• bun typecheck in packages/tui
• Pre-push bun turbo typecheck --concurrency=3: 33 tasks successful
• Earlier TUI suite during implementation: 584 passed, 5 skipped
• Live-server smoke check with bun run dev:live against the elected V2 service

Demo

The capture below shows the source-built TUI connected to the live V2 service, with vertical tabs, project metadata, active/unread states, and title-edge fading.

Flow

flowchart LR
Config[Tabs setting] -->|vertical off| Horizontal[Horizontal tab strip]
Config -->|vertical on| Rail[Vertical tab rail]
Sessions[Session tab state] --> Rail
Status[Run and unread status] --> Pulse[Independent pulse channels]
Pulse --> Rail
Rail --> Layout[Rail-aware session layout]

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