[Feature Request] Improve discoverability of session history in Agent View: peek vs attach unclear, Ctrl+O not hinted

Bug Description
[Agent View UX] History is hard to reach: peek-vs-attach is unclear and reading past output requires discovering Ctrl+O

Summary

Reading a background session's earlier output from Agent View is currently
hard to discover. Two friction points make the history feel unreachable,
and the fullscreen renderer (mouse-wheel chat scroll) can't be enabled for
background sessions.

### Friction points

1. Space (peek) vs Enter/→ (attach) is not discoverable.

The footer shows "enter to open · space to reply". Space opens the peek
panel, which by design shows only the most recent output (the tail).
It's easy to assume that tail is the whole session and conclude the
history is truncated. Nothing signals that the full conversation lives
behind Enter/→ (attach), not Space.

1. After attaching, viewing earlier output requires knowing about Ctrl+O.

On attach you see a recap + recent messages, and scrolling up does not
reveal earlier history in the default view, so it looks like the history
ends. Only Ctrl+O (transcript mode) shows the full conversation, with
arrow/PgUp/PgDn (and mouse wheel) scrolling. This is not hinted anywhere
in the attached view.

1. Fullscreen renderer unavailable in background sessions.

/tui fullscreen returns "Renderer switching isn't available in a
background session — press ← to detach and run /tui from a foreground
session", and "tui": "fullscreen" in settings.json is ignored for
sessions viewed via Agent View. So mouse-wheel-scrolls-chat only works
once you're in transcript mode (Ctrl+O) or after resuming foreground.

### Requested improvements

• Make peek vs attach clearer (e.g. peek panel hint: "showing latest output

— press Enter to open full conversation").

• In the attached view, hint that Ctrl+O opens the full scrollable

transcript when the user scrolls up / hits the top of the visible buffer.

• Ideally honor "tui": "fullscreen" (or an equivalent) for attached

background sessions so mouse-wheel chat scroll works without Ctrl+O.

### Current workaround (works, but not obvious)
Select row → Enter (attach, not Space) → Ctrl+O (transcript) →
scroll with ↑/↓, PgUp/PgDn, or mouse wheel; / to search, q/Esc to exit.

### Environment

• Claude Code: 2.1.150
• OS: macOS 26.5 (build 25F71)
• Terminal: iTerm2 3.6.10
• settings.json: "tui": "fullscreen" set (ignored for Agent View sessions)

### Related

• #12953 (mouse wheel scrolls input history instead of chat)
• #5368 (cannot scroll up to view conversation history)

Environment Info

• Platform: darwin
• Terminal: xterm-256color
• Version: 2.1.150
• Feedback ID: 0e0b1793-604b-4387-b113-404152f3ad36