Remove duplicate src/megadetector_ai package (canonical is megadetector_core)

[rain-Brian]

Summary

The repo currently tracks two Python packages under src/ with identical file sets:

• src/megadetector_core/ — canonical; the installed entry point is megadetector = "megadetector_core.cli:main" (see pyproject.toml, name = "megadetector-core").
• src/megadetector_ai/ — a duplicate (__init__.py, cli.py, detector.py, training.py, training_utils.py) that is not referenced by the project entry point.

Why this matters

• The two copies can silently diverge; bug fixes/edits to one won't reach the other.
• Contributors and documentation can reference the wrong namespace.
• Increases package size and confusion for anyone reading src/.

Proposed action

• Confirm megadetector_ai has no remaining importers/entry points.
• Remove src/megadetector_ai/ (or, if it is intentionally retained, document why and de-duplicate the shared code).

Notes

Surfaced during a documentation/SEO pass; all new docs reference megadetector_core only. Tracking-only issue — no code change required from the docs work.