Add safe one-command Cursor MCP registration

[Singularity-tian]

Problem

Persome can generate a generic stdio mcpServers.persome object, but Cursor users must merge it manually. The Runtime already has idempotent installers for Claude Code, Codex, Claude Desktop, and opencode.

Scope

Add persome install cursor and persome uninstall cursor without overwriting unrelated servers or malformed user configuration.

Acceptance criteria

• Supports project .cursor/mcp.json and user ~/.cursor/mcp.json, with an explicit precedence rule.
• Writes an absolute Persome executable path plus mcp args.
• Preserves unrelated keys and other MCP servers.
• Install and uninstall are idempotent.
• Tests use an isolated HOME and never modify a developer's real Cursor config.
• docs/mcp-clients.md, CLI help, and uninstall documentation are updated in the same PR.
• Offline CI, PII, secret, language, and documentation-link gates pass.

Pointers

Start with the client installers in src/persome/cli.py and their isolated config-mutation tests. Keep Cursor-specific behavior out of the daemon and model layers.