Add "Always allow globally" option to permission prompts

When Claude Code surfaces a permission prompt for a tool (especially MCP tools), the current options are:

• Allow once
• Always allow in this project (writes to .claude/settings.json)
• Deny

There's no way to allow a tool globally across all projects from this prompt. The only path today is to manually edit ~/.claude/settings.json after the fact — or open a new session and repeat the process there.

Proposed change

Add a fourth option: "Always allow globally" — which writes the permission rule to ~/.claude/settings.json instead of the project settings file.

Why this matters

MCP tools like Jira, Google Calendar, Slack, and GitHub are used across every project, not just one. Having to re-approve them per project, or maintain the global allowlist by hand, creates unnecessary friction. The project-level option makes sense for project-specific tools; a global option is the natural complement for user-level tools.

Expected behavior

Selecting "Always allow globally" writes the tool name to ~/.claude/settings.json under permissions.allow, identical to what "Always allow in this project" does for .claude/settings.json.