[Feature Request] Jump between conversation turns with keyboard shortcuts

Bug Description
Feature Request: Jump between conversation turns (like iTerm2 shell integration)

When working with Claude Code in long sessions, conversations can become very lengthy. Currently, the only way to navigate is basic
scrolling (PageUp/PageDown, Ctrl+Home/End), which makes it difficult to quickly locate a specific exchange.

iTerm2 solves an analogous problem with its shell integration: pressing Cmd+Shift+Up/Down lets users jump directly between previously
entered commands, skipping over all the output in between. This is extremely useful for orienting yourself in a long terminal session.

Proposed feature: Add keybindings (e.g. Ctrl+Shift+Up / Ctrl+Shift+Down) that jump the scroll position directly to the previous/next user
message (conversation turn), skipping over assistant responses and tool output in between.

Why this matters:

• In long sessions with many turns, scrolling through verbose tool output and code blocks to find a specific prompt is tedious
• The existing Esc+Esc rewind dialog is close but designed for rewinding/summarizing, not lightweight navigation
• This is a well-established UX pattern (iTerm2, Warp, VS Code terminal) that users already expect

Suggested implementation:

• New scroll context actions: scroll:previousTurn and scroll:nextTurn
• Default bindings: Ctrl+Shift+Up / Ctrl+Shift+Down (or Cmd+Shift+Up/Down on macOS)
• Customizable via ~/.claude/keybindings.json

Environment Info

• Platform: darwin
• Terminal: iTerm.app
• Version: 2.1.98
• Feedback ID: 12876374-ca23-4bd3-9496-3c6fbf6b30b1