Disclose usage reporting: README, environment opt-out, notice link

[dmccoystephenson]

gh-backup has reported startup and backup-completed events to trace since #22, with a first-run notice that named only its own opt-outs and no link to what trace collects. The vendored trace client is updated to 0.2.0, which honours TRACE_USAGE_REPORTING=off and DO_NOT_TRACK=1 in the environment before the usage.reporting.enabled property gh-backup already passes through the builder; the first-run notice stays first-run but now names TRACE_USAGE_REPORTING=off and links to the details page, and the README's usage-reporting section becomes a top-level ## Usage reporting listing every opt-out. close() still flushes the one event before a short CLI run exits. Nothing about what is sent changes.

• src/main/java/com/github/backup/trace/TraceClient.java, src/test/java/com/github/backup/trace/TraceClientTest.java: re-vendored from trace-client-java 0.2.0, package line only changed (serverWideConfig is Spigot-only and is not used).
• src/main/java/com/github/backup/UsageReportingService.java: the first-run notice carries the details link and TRACE_USAGE_REPORTING=off; the class comment lists the environment opt-outs.
• src/test/java/com/github/backup/trace/UsageReportingEnvironmentTest.java (new), src/test/java/com/github/backup/UsageReportingServiceTest.java: DO_NOT_TRACK=1 and TRACE_USAGE_REPORTING=off are asserted to yield isEnabled() == false even when the property says enabled; the notice is asserted to contain the new opt-out and the link.
• README.md, CONFIG.md, .env.example, docker-compose.yml, src/main/resources/application.properties, CHANGELOG.md: ## Usage reporting per the shared wording (events, what is not sent, every opt-out, details link); TRACE_USAGE_REPORTING and DO_NOT_TRACK are documented alongside USAGE_REPORTING_ENABLED and passed through from .env by the Docker daemon, empty by default.

🤖 Generated with Claude Code

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drafted by Claude on behalf of Daniel Stephenson