feat: Default CLI Authentication to RDA Flow

[KCarretto]

This submission changes the default CLI authentication mechanism from a local browser-based OAuth flow to a Remote Device Authentication (RDA) flow.

Users typically run Tavern CLI tools on remote systems (like SSH sessions) where local browser redirection is impossible. The previous default browser flow would often fail in these environments and then fallback to RDA. Making RDA the explicit default behavior removes this unnecessary failure path and improves the user experience.

The original browser flow is preserved and can be enabled by setting TAVERN_USE_BROWSER_OAUTH=1. Docs and tests have been updated appropriately.

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PR created automatically by Jules for task 9050903838038517202 started by @KCarretto

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[github-actions]

Summary

Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
2896	2896	0	0	0	0	1ms

Previous Results

Build 🏗️	Result 🧪	Tests 📝	Passed ✅	Failed ❌	Skipped ⏭️	Other ❓	Flaky 🍂	Duration ⏱️
#1917	✅	2896	2896	0	0	0	0	32.3s

Insights

Average Tests per Run	Total Flaky Tests	Total Failed	Slowest Test (p95)
2896	0	0	5m 46s

Test Changes

0 test added, 0 removed

Slowest Tests

Test 📝	Results 📊	Duration (avg) ⏱️	Duration (p95) ⏱️
eldritch-libfile: std::list_recent_impl::tests::test_list_recent_default_args	3	4m 12s	5m 46s
eldritch-libfile: std::list_recent_impl::tests::test_list_recent_default_args	3	4m 12s	5m 46s
eldritch-libfile: std::list_recent_impl::tests::test_list_recent_default_args	3	4m 12s	5m 46s
TestDockerExecutor_Build_ContextCancellation	1	5.5s	5.5s
eldritch-libsys: std::dll_inject_impl::tests::test_dll_inject_simple	1	5.2s	5.2s
TestInteractiveShell	1	5.1s	5.1s
TestOtherStreamOutput	1	5.0s	5.0s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s
imix::bin/imix: tests::task_tests::test_task_streaming_output	3	3.0s	3.0s

_{🎉 No failed tests in this run. | 🍂 No flaky tests in this run.}

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