feat(tui): add inline Ctrl+R reverse-i-search for prompt history

[edlsh]

Summary

Implements bash-style inline reverse-i-search for prompt history, as an alternative approach to #5775's modal dialog. This addresses #5062 and #1701.

As @ShpetimA suggested in #5062:

Maybe just simple search style like bash where you search and it shows on the input instantly instead of opening history modal.

Demo

The search works inline in the prompt textarea with a status line in the footer:

(reverse-i-search)'git co': 2/5    ctrl+r next    esc cancel    enter select

Features

• Inline preview: Matches display directly in the prompt textarea (no modal)
• Incremental search: Real-time substring matching as you type
• Full keyboard controls:
  • Ctrl+R / Up: Cycle to older matches
  • Down: Cycle to newer matches
  • Enter: Accept current match
  • Esc / Ctrl+G: Cancel and restore original prompt
• Complete state preservation: Prompt text, file/agent parts, extmarks, mode, and cursor position
• Configurable keybind: history_search (default: ctrl+r)

Implementation Details

• Added historySearch state to Prompt store tracking: active, query, matchIndex, originalPrompt, originalMode, originalCursorOffset
• Exposed history.items getter for direct access to prompt history
• History search handler takes precedence over shell-mode and other key handlers when active
• Preview updates parts/extmarks consistently with normal history navigation
• Paste blocked during search mode
• Supports space character in search queries (e.name === "space" handling)

Why Inline vs Modal?

Aspect	Inline (this PR)	Modal (#5775)
UX Convention	Matches bash/zsh/fish exactly	IDE-style picker
Context	Stays in prompt, see full content	Separate overlay
State Management	Reuses existing textarea/extmarks	Needs separate rendering
Complexity	Localized to one component	Additional dialog component

For a terminal TUI that already uses shell-like behaviors, inline Ctrl+R is the more cohesive choice.

Closes #5062 Related to #1701, #5775