feat: persistent tab strip with animated status during attach

[andreano-che]

Summary

Adds a persistent horizontal tab strip that stays visible at the top of the terminal when attached to a session. No more detaching just to see what other sessions are doing or to switch between them.

• Horizontal tab bar with animated status indicators (braille spinner for running, static ◐ for waiting)
• Alt+1–9 instant tab switching without leaving the current session (zero flash)
• Read/unread tracking via existing acknowledged field — completed tasks glow until you look at them
• Fully configurable via config.toml ([tab_strip] section)

How it works

When you press Enter to attach, agent-deck creates a narrow tmux split pane at the top running a lightweight tab-strip process. This process reads session state from SQLite and renders animated status icons. Alt+N key bindings are registered at the tmux level, so they work even while typing in the session.

Screenshots

Tab strip visible during active session:

Switching between tabs with Alt+N:

New In Progress live animation

Main screen (unchanged):

New files

File	Purpose
internal/ui/tab_strip.go	TabStripModel — rendering, animations, status icons, dual layout (vertical/horizontal)
internal/ui/tab_strip_test.go	Unit tests (10 tests)
internal/ui/tab_strip_app.go	Standalone Bubble Tea app for the tmux split pane process
cmd/agent-deck/tab_strip_cmd.go	agent-deck tab-strip subcommand
cmd/agent-deck/tab_switch_cmd.go	agent-deck tab-switch N subcommand (called by tmux key bindings)

Modified files

File	Change
internal/ui/home.go	Split-pane setup in attachSession(), switch handling, cleanup
internal/ui/hotkeys.go	Tab navigation hotkey constants (tab_1–tab_9, next_tab, prev_tab)
internal/session/userconfig.go	TabStripSettings struct with TOML parsing
cmd/agent-deck/main.go	Register new subcommands

Configuration

[tab_strip]
enabled = true              # master switch (default: true)
layout = "horizontal"       # "horizontal" or "vertical"
width = 15                  # column width for vertical layout
show_hotkey_hints = true    # show 1, 2, 3 next to tabs

Keyboard shortcuts

Action	Default	Configurable
Switch to tab N	Alt+1–Alt+9	Yes, via [hotkeys]
Next tab	Alt+]	Yes
Previous tab	Alt+[	Yes
Toggle unread	u	Already existed

Design decisions

• Separate process for tab strip: The tab strip runs as a lightweight agent-deck tab-strip process in a tmux split pane, not inside the main Bubble Tea app. This is necessary because tea.Exec (used for tmux attach) takes over the terminal — the main app can't render during attach.
• tmux-level key bindings: Alt+N bindings are registered via tmux bind-key -T root, so they intercept keys before they reach the application inside the pane. This means they work while typing in any session.
• Zero-flash switching: When switching tabs, isAttaching stays true so View() returns "" throughout the transition — no flash of the main screen.
• Read-only SQLite access: The tab strip process opens the database read-only and polls every 1 second. No write contention with the main process.
• Non-breaking: The main screen is completely unchanged. The feature is opt-out via [tab_strip] enabled = false.

Testing

• 10 unit tests for TabStripModel (selection, animation, rendering, transitions)
• 2 unit tests for TabStripSettings config parsing
• Manual testing: attach, switch tabs, detach, verify cleanup

Test plan

• Build and run agent-deck with 3+ sessions
• Enter a session — verify tab strip appears at top
• Press Alt+2 — verify instant switch without flash
• Press Ctrl+Q — verify clean return to main screen (no leftover panes)
• Set [tab_strip] enabled = false — verify tab strip does not appear
• Verify no impact on existing session list, preview, or hotkeys