feat(tui): add plus button to session tab bar - #41887

What

Adds a plus affordance to the session tab bar so a new tab can be opened with the mouse, matching browser tab strips. Previously the only way to open a new tab was the session.new command or navigating home via keybinds.

The plus and the temporary new-session tab are one control with two states: a subdued + affordance that promotes in place into the active new-session slot when clicked, then demotes back (or becomes a real numbered tab once a prompt is submitted). There is never a separate pseudo tab plus a redundant + at the same time.

• Horizontal strip: a subdued + sits at the end of the strip (hover lifts it like a tab). Clicking it grows the slot into the active tab — bold + New session on the selected background, with the + glyph where the number would be, and a close × on hover. While promoted, the trailing plus is hidden. On submit, the slot becomes a real numbered tab and a fresh + reappears after it.
• Vertical rail: a + New session row renders below the tab list. Same promotion in place: bold, blue +, selected background, hover close ×. Real tabs never share the rail with a "New session" pseudo tab anymore.
• Opening carries the current session's location so the new session lands in the same project, exactly like the session.new command.

How

• packages/tui/src/context/session-tabs.tsx — new add() action on the session-tabs context: navigates home with the current session's location, falling back to location.ref. Same semantics as the session.new command in app.tsx.
• packages/tui/src/component/session-tabs.tsx — optional add on SessionTabsController.
  • Horizontal: the promoted NEW_SESSION_TAB still joins the adaptive layout as the active tab (so the existing width-growth animation carries the + stretching into the tab), but renders + in the number cell; showPlus() hides the trailing affordance while promoted and drops its ADD_TAB_WIDTH (3) reservation.
  • Vertical: the pseudo tab is gone from the list entirely; the affordance row itself renders the promoted state (newTab()), and the rail scrolls its bottom into view while promoted.
• packages/tui/src/feature-plugins/system/storybook/session-tabs.tsx — storybook controller wires add to the existing fixture add-tab logic (shared with the t keybind).

Flow

flowchart LR
    idle["subdued + affordance"] -->|click| promoted["slot promotes in place:\nbold + New session, selected bg"]
    promoted -->|submit prompt| real["real numbered tab"]
    real --> idle2["fresh + appears after it"]
    promoted -->|close × / switch away| idle

Scope

• No new keybind: session.new already covers keyboard access; this is mouse parity for the tab strip.
• Follow-ups discussed but not in this PR: reviving the title wipe animation for background title updates, and dimming the placeholder title on not-yet-named sessions.

Testing

• bun typecheck (packages/tui) clean.
• bun run test (packages/tui): 642 pass, 0 fail.
• New test: add opens the new session tab carrying the current session's location in test/context/session-tabs.test.tsx.
• Manually verified both orientations in a real PTY via terminal-control: idle affordance, hover, promotion on click, and the promoted slot rendering with no second + anywhere.

Demo

OpenCode Drive recording (simulated LLM): a session starts from the promoted new-session slot, the + is clicked and promotes into the active tab, a second session starts there, and clicking tab 1 switches back — the slot demotes to the subdued +.