Add telemetry opt-out controls and first-run notice

[pythrick]

This PR tightens telemetry transparency and user control.

Changes:

• add config opt-out via telemetry = false (also supports telemetry.enabled = false and [telemetry] enabled = false)
• add CLI session opt-out via --no-telemetry
• only call telemetry ping when enabled
• show a one-time first-run notice with disable instructions
• fix telemetry docs to match actual request method/payload (POST JSON)
• document disable options in CLI/config docs
• add tests for config + flag behavior

Fixes #102