[FEATURE] VS Code extension: cap chat panel max-width for readability on wide displays

Problem

The Claude Code VS Code extension's chat/conversation pane renders text at the full width of its panel. On wide monitors (4K, ultrawide) — or even on a standard display when the extension panel is dragged wide — message lines stretch edge-to-edge and become hard to read. Reading-width UX research consistently lands around 50–75 characters per line; the current behavior blows past that.

Why this can't be worked around today

• No claudeCode.chatMaxWidth / messageWidth / similar setting exists in either VS Code settings.json or ~/.claude/settings.json.
• The VS Code webview API sandboxes the extension's iframe, so Custom CSS / JS Loader extensions cannot reach into the chat panel to apply a max-inline-size constraint from the outside.
• The only "workaround" is to manually narrow the panel by dragging the sidebar — fighting the layout instead of using it.

Request

Add a setting (e.g. claudeCode.chatMaxWidth) that constrains the message column's inline size. Suggested shape:

• Default: ~50em / ~800px (a reasonable reading width).
• Accepts a CSS length (em, ch, px, %) or the literal none to opt out and restore current full-width behavior.
• Applied as max-inline-size on the message column so it composes with VS Code's existing layout.

A simple toggle (narrow / wide / full) would also be a strong v1.

Prior issues

• #48171 — same request, auto-closed by the stale bot with "open a new issue if this is still relevant." Filing this per that suggestion.
• #57991 — open request for the same behavior on the Desktop app's Code tab; this is the VS Code extension sibling.

Environment

• Claude Code VS Code extension (current)
• macOS, wide display