chore: remove demo mode

[jdwit]

Summary

Demo mode (YTSTUDIO_DEMO=1) was a parallel codepath that intercepted get_data_service / get_analytics_service and substituted hand-rolled fakes backed by bundled JSON fixtures. It was useful early on for showing the CLI without OAuth, but it has stopped earning its keep:

• the fixtures drift from the real YouTube Data / Analytics API shape, so demo runs are only loosely representative of the real surface;
• new commands (the videos upload pipeline) silently bypass the demo service entirely, so partial coverage is itself misleading;
• users get confused about whether a successful demo run means anything for their real channel.

Rip it out wholesale.

What changes

• src/ytstudio/demo.py and src/ytstudio/demo_data/ deleted.
• src/ytstudio/services.py drops the is_demo_mode branches; both factories always build the authenticated client.
• tests/test_analytics.py drops the demo-only TestQueryDemoMode class and the demo-patched test_overview; remaining coverage exercises every command path against mocks.
• docs/videos.md and docs/livestreams.md drop their "Demo mode" sections.
• Incidental demo labels in test fixtures (MOCK_VIDEO tags, sidecar filenames and descriptions) renamed to sample so no test reads like it is testing the (now nonexistent) demo path.

Test plan

• uv run pytest -q -> 195 passed, 80% coverage
• uv run ruff check . -> clean
• uv run mkdocs build --strict -> clean
• grep -rni demo docs/ src/ tests/ README.md mkdocs.yml -> no hits