feat: anonymous opt-out usage telemetry

[a-essawy]

Summary

Anonymous, opt-out usage telemetry so we can see which commands matter and make the CLI better.

• One cli_command event per run: command name, success, duration, CLI version, OS. Nothing else.
• Never sends file names, arguments, URLs, credentials, or account identity. Identifier is a random per-machine id (cli_anon_*), not tied to any account. Events are anonymous ($process_person_profile: false).
• rb telemetry on|off|status; env off switches DO_NOT_TRACK=1 / RENDOBAR_TELEMETRY=0; auto-off in CI.
• Respectful one-time first-run notice; README "Telemetry" section documents exactly what is and isn't collected.
• First-party: posts through the e.rendobar.com proxy. Public write-only token injected at build via RB_POSTHOG_KEY (empty = disabled).

Verification

Full suite green (174 tests), typecheck clean. Verified end-to-end against a local capture server: anonymous cli_command posted to /i/v0/e/, and DO_NOT_TRACK=1 / rb telemetry off send zero requests.

To activate on the next release (repo owner)

Because the token is baked in at build time, add a repository variable RB_POSTHOG_KEY (Settings, Actions, Variables) and add to the "Generate version file" step in .github/workflows/cli-binaries.yml:

        env:
          RB_POSTHOG_KEY: ${{ vars.RB_POSTHOG_KEY }}

(Left out of this PR because the automation token lacks workflow scope.)

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