Separate mcp_root from cwd so tool-arg overrides don't clobber the
session-level root from initialize. Fallback chain matches RFC spec:
cwd → mcp_root → walk tree → fail with guidance. Error messages now
include attempted paths and actionable fix suggestions. Added --debug
flag to MCP server for file-based DEBUG logging.
Phase 2 finding: Claude Code v2.1.19 declares roots capability but
sends no roots array. Walk-up is the primary detection path.
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
