feat(mcp): add a typed PostHog wrapper module for packages/loopover-mcp (local, opt-in)

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Context

Part of #6228 (telemetry spec). The local --stdio server needs its own wrapper, separate from the remote server's (per #6228: the local CLI's telemetry must default to opt-in, not opt-out, since it runs on a user's own machine — a materially different trust posture than the hosted remote server).

Requirements

• Add the PostHog Node SDK as a dependency of packages/loopover-mcp.
• Create a small wrapper module exposing the same shape of function as the remote-side wrapper (recordMcpToolCall({ tool, callerType: "local", ok, durationMs })), for consistency between the two.
• The wrapper must check a persisted opt-in flag (see the companion telemetry enable/disable CLI-command issue) before sending anything — default OFF until a user explicitly opts in.
• Must be a safe no-op if telemetry isn't enabled or the network call fails — never affect the CLI's actual command behavior.

Test Coverage Requirements

99%+ Codecov patch coverage; test both the opted-in and default-off (safe no-op) paths.

Deliverables

• A typed wrapper module in packages/loopover-mcp, mirroring the remote side's shape.
• Default-off behavior verified.

Expected Outcome

The local CLI has a ready-to-use, opt-in-gated telemetry function for the tool-dispatch instrumentation issue to call.

Links & Resources

• Spec: usage-telemetry instrumentation strategy for MCP (PostHog) #6228 (parent spec)
• The companion telemetry enable/disable CLI-command issue (provides the opt-in flag this reads)

[linear-code]

JSO-4324