docs(17): research custom bind mounts phase

Phase 17: Custom Bind Mounts
- Bollard MountTypeEnum::BIND for bind mount implementation
- Path validation with std::fs::canonicalize()
- CLI subcommand pattern from user/mod.rs
- Config schema extension for mounts array
- Common pitfalls documented (Windows paths, file sharing)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: pRizz