Feature: live reply preview, scrollable approval diffs, a context meter, and /history in the CLI - #101

Summary

Four additions to the CLI that make long replies, long diffs and long sessions easier to follow:

1. Live reply preview: while a reply streams, its latest lines show dimmed above the spinner.
2. Scrollable diffs in approvals: a long diff shows in a scrollable window above the approve/deny menu instead of pushing the menu off screen.
3. Context meter: a bar above the prompt shows how full the model's context window is.
4. /history: browse this session's conversation, filtered as you type.

What changed

• Live reply preview: the last five lines of the streaming reply appear dimmed above the spinner and update at most once per spinner frame. They disappear with the spinner, and the finished reply still prints as formatted markdown. Lines are fitted to the terminal width so a wrapped line can't throw off the redraw.
• Scrollable diffs: a change over 30 lines that needs a decision now leaves a one-line summary in scrollback. The diff itself appears in a fixed-height window above the menu, scrolled with PgUp/PgDn/Home/End, while ↑/↓ and Enter still choose. Short diffs and auto-approved changes print as before.
• Context meter: shows the last request's size against the model's context window, e.g. Context ████████░░░░ 17k / 32k (54%). It turns yellow at 60% and red at 85%, with the empty part drawn in grey so the bar's length is easy to read. Cloud models manage their window server-side, so they show the token count only.
• /history: opens the session's messages in a scrollable window with a search box. Tab moves between the search box and the list; Esc, or Enter on an empty search, closes it.

Why this was done

• A long reply used to look like a frozen "Thinking…" until it finished.
• A long diff could push the approval menu hundreds of lines down, so reviewing a large change meant scrolling back and forth.
• On small local models, the context window fills quickly and the model then starts losing the earlier conversation. The meter shows when to run /compact before that happens.
• There was no way to look back through a long session without scrolling the terminal.

How it was tested

• New unit tests for the preview layout (wrapping, wide characters, control characters) and the context meter math (levels, clamping, cloud models). The full harness test suite passes on .NET 8 and .NET 10.
• Checked in the CLI: the context meter renders and updates after each reply.
• Not yet exercised by hand: PgUp/PgDn in a long diff approval, and Tab/Esc in /history. These depend on how RazorConsole reports those keys, so they're worth a quick manual check.

Notes

• The meter and /history only render while the CLI is idle, and the diff window only while an approval menu is up. RazorConsole redraws its area in place, so it can't share the screen with output printed while a request runs. The live preview uses the spinner's existing redraw technique for the same reason.
• MandoCode Desktop reads the CLI's slash-command list for its autocomplete, so /history will show there once Desktop picks up this engine version. Desktop has no handler for it and reports it as unknown; Desktop's transcript already has Ctrl+F search.
• Changelog: entries added under [Unreleased] → Added.