feat(mcp): add `loopover-mcp telemetry enable/disable` CLI command + persisted opt-in flag

[JSONbored]

Context

Part of #6228. The local MCP server's telemetry must default OFF and require an explicit opt-in (per #6228's decision) — this issue builds that switch.

Requirements

• Add a loopover-mcp telemetry enable / loopover-mcp telemetry disable command pair, following this CLI's existing command conventions (see loopover-mcp login/logout for the shape of a similar state-toggling command).
• Persist the opt-in state in the same config file loopover-mcp login already uses (loopoverConfigPath/loadLoopoverConfig convention in github-token-resolution.js, or the CLI's own config module if different) — a new top-level telemetryEnabled: boolean field, default false/absent.
• loopover-mcp status/doctor should report current telemetry opt-in state, matching how they already report auth/session state.
• Add --json support, matching every other command in this CLI.

Test Coverage Requirements

Match this package's existing test conventions for CLI commands; test both toggling and the default-off state.

Deliverables

• telemetry enable/disable commands implemented.
• status/doctor reflect the current opt-in state.
• Config persists across CLI invocations.

Expected Outcome

A user can explicitly and durably opt in (or back out) of local MCP telemetry, with clear visibility into their current state.

Links & Resources

• Spec: usage-telemetry instrumentation strategy for MCP (PostHog) #6228 (parent spec)
• packages/loopover-mcp/bin/loopover-mcp.js's existing login/logout/status/doctor commands (the conventions to match)

[linear-code]

JSO-4327