feat(cli): add consent-gated runtime telemetry for NAT CLI commands

[yczhang-nv]

Description

Summary

Adds aggregate, anonymous CLI-usage telemetry to the nat Python CLI. Modeled on DataDesigner's telemetry.py and piggybacks on the existing NeMo Usage Telemetry project (clientId=184482118588404, schema 1.4 — companion MR in aire/microservices/nemo-telemetry).

Posture is consent-gated, not default-on. A first-run prompt asks the user whether to allow telemetry; the decision is persisted. Non-interactive (CI / cron / piped) sessions never prompt and default OFF. NAT_TELEMETRY_ENABLED env var overrides both.

Note: The default ingest endpoint is intentionally blank in this PR — no telemetry will be sent over the network until an endpoint URL is configured (planned for a small follow-up PR after the schema MR lands and the GX Telemetry endpoint is confirmed). Until then, the code path is exercised but no traffic leaves the machine. Local debugging via NAT_TELEMETRY_ENDPOINT=stdout writes payloads to stderr.

Consent flow

Order of precedence for whether telemetry is active:

1. NAT_TELEMETRY_ENABLED env var (any value — 1/true/yes enables, anything else disables). Sets the answer for this shell session and bypasses the prompt.
2. Persisted decision at ~/.config/nat/telemetry.toml (TOML, written once when the user answers the first-run prompt; can be re-set anytime via nat configure telemetry --enable | --disable).
3. Interactive prompt (TTY-only) on first run when neither of the above is set. Default answer is no (enter, EOF, or Ctrl-C all yield "disabled"). The prompt explicitly lists what is collected and what is not.
4. Default OFF in non-interactive sessions (no TTY → no opportunity to ask → no data sent).

The persisted decision is at ~/.config/nat/telemetry.toml:

[telemetry]
consent = "enabled"
consented_at = "2026-04-30T21:21:04Z"
prompt_version = "1.0"

CLI subcommand: nat configure telemetry
Three modes:

• nat configure telemetry --enable — persist consent as enabled
• nat configure telemetry --disable — persist consent as disabled
• nat configure telemetry --status (default) — show current effective state and its source

The status command also flags when an env var is overriding the persisted decision, so users aren't surprised when their persisted choice doesn't take effect.

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• When the PR is ready for review, new or existing tests cover these changes.
• When the PR is ready for review, the documentation is up to date with these changes.

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