#2831 Telemetry opt-out scope: one user-level setting that disables telemetry everywhere

Split out of swamp-club#2817 (verified by that issue's audit):

• .swamp.yaml telemetryDisabled is honoured inside a repo but not for commands run outside it.
• The user-level telemetry.yaml disabled setting is honoured outside repos but not inside them.
• A repo-dir that does not resolve, and any command run outside the repo, fall back to the public default endpoint even when the repo's .swamp.yaml sets telemetryEndpoint or telemetryDisabled.

Proposal: a user-level opt-out wins everywhere; consider honouring DO_NOT_TRACK; document a supported local-only mode (for example telemetryEndpoint none: spool locally, never send). Document every opt-out and its scope (see swamp-club#1597).