chore: remove config_explorer/ directory after migration

[anfredette]

Summary

After Config Explorer code, data, and tests have been moved into the unified project structure, remove the config_explorer/ directory.

Scope (Steps 7-8 from migration plan)

Move remaining files (Step 7)

• config_explorer/README.md → docs/CONFIG_EXPLORER.md
• config_explorer/empirical-vllm-memory-results.md → docs/
• config_explorer/examples/ → examples/config_explorer/ (or remove)

Delete directory (Step 8)

• Remove entire config_explorer/ directory
• Verify no remaining references to config_explorer/ paths in the codebase

Prerequisites

• #TBD (move Config Explorer core logic)
• #TBD (integrate Config Explorer UI)

Reference

See migration plan PR #129 Steps 7-8.