Fix #298: Clarify drift baseline agent scope and live session rules

[tarun73]

Explain that behavioral drift baseline thresholds are computed per-agent (rather than globally) and build only from live sessions. Update the empty page warning on the Web UI and the CLI fallback message to explicitly state this.

Summary

• Web UI: Updated the Drift View empty-state warning message in tokenjam/ui/index.html to clarify the per-agent live threshold.
• CLI: Updated the fallback warning printed by tj drift when no baselines exist in tokenjam/cli/cmd_drift.py to match the Web UI message.

Tests / Verification

• Added test_drift_no_baselines_warning_message in tests/integration/test_cli.py to verify that tj drift prints the warning header when no baselines exist.
• Executed the full test suite successfully (1,154 passed).

What's NOT in this PR (if scope was deliberately limited)

• Phase 2 (Backfill baseline compilation): Auto-building baselines during tj backfill is deferred to a future follow-up issue/PR.

Related issue

Closes #298

@anilmurty please review

Checklist

• Tests pass (pytest tests/unit/ tests/synthetic/ tests/agents/ tests/integration/)
• Lint clean (ruff check tokenjam/)
• Type check clean (mypy tokenjam/)
• CLAUDE.md updated (if architecture changed)
• Test spans use tests/factories.py (not raw NormalizedSpan)
• Requested @anilmurty as reviewer (or @-mentioned him above)

[anilmurty]

Thanks @tarun73 — exactly the clarity fix for #298, and on both surfaces. The new message states the two things that confused the original (per-agent threshold, and live-only so backfilled sessions won't build a baseline), and CLI + Web UI now match. Good scope call deferring the backfill-baseline compilation to a follow-up — that was the bigger half of #298 and is rightly separate. Test asserts the warning, module JS clean, all four CI green.

Ready to merge.