Allow suppressing native installer notification for npx/npm users

Problem

When running Claude Code via npx (npx @anthropic-ai/claude-code@latest), a persistent notification appears on every startup:

Claude Code has switched from npm to native installer. Run `claude install` or see https://docs.anthropic.com/en/docs/claude-code/getting-started for more options.

This cannot be dismissed or suppressed. In narrow terminal panes (e.g. 4-pane splits in Ghostty), the message wraps badly into a single narrow column, causing visual disruption before disappearing.

Why users stay on npm/npx

• Version control: npm install -g @anthropic-ai/claude-code@2.1.34 lets you pin and roll back. Native auto-updates don't.
• Avoiding conflicts: Dual npm+native installs cause version mismatches and crashes (see #22415, #10280, #21789).
• Latest channel via npx: npx @anthropic-ai/claude-code@latest auto-resolves to newest version without native auto-updater.

Request

Add a way to suppress this notification. Options:

1. A settings.json flag: "suppressNativeInstallerNotice": true
2. An env var: CLAUDE_SUPPRESS_INSTALL_NOTICE=1
3. A one-time dismiss: claude config set install-notice false

Any of these would work. The notification is informational, not critical — users who've chosen npm/npx have done so deliberately.

Environment

• macOS 15 (Darwin 25.2.0)
• Claude Code 2.1.34 via npx
• Terminal: Ghostty with 4-pane vertical splits (~70 columns per pane)