fix(fork): Lathe sends no usage telemetry unless opted in

[jamesainslie]

Lathe inherited upstream's product telemetry unchanged: the server posted usage events to T3 Tools' PostHog project, with T3's project key, on by default, and the welcome screen and Settings told users "T3 Code collects anonymous usage data" under T3's privacy policy. A fork should not report its users to upstream's analytics by default, and the copy read as if Lathe's maker collected the data.

Change

• FORK_IDENTITY.telemetryEnabledByDefault is false, and AnalyticsService reads it as the default for T3CODE_TELEMETRY_ENABLED (one import plus a one-line swap in the upstream file). Setting the variable to true still opts in.
• The welcome screen and the Settings privacy row now say Lathe sends no usage data. The privacy policy link stays, because T3 Connect and the other T3 services Lathe uses follow T3 Tools' policy.
• docs/user/telemetry.md describes the opt-in instead of the opt-out.

Verification

• New test: with T3CODE_TELEMETRY_ENABLED unset, nothing is sent. It failed before the change (one batch posted) and passes after; the other four telemetry tests still pass.
• forkIdentity.test.ts passes; shared, server and web typecheck; lint and format pass on the changed files.