[codex] Extract project config parser

[sabino]

Summary

• Move dbt_project.yml loading and project path/docs config parsing into src/project/config.zig.
• Keep src/project.zig as the graph/parser facade by importing the config loader through a narrow alias.
• Move config-focused native Zig coverage into the new module and update PLAN.md current status.

Validation

• zig fmt --check src/project.zig src/project/config.zig src/project/util.zig
• zig build
• zig build test
• pytest -q tests/test_cli.py -> 48 passed
• pytest -q -> 52 passed
• python scripts/check_runtime_boundary.py
• python scripts/check_public_safety.py
• git diff --check
• Inline codex exec preplan saved under ignored .agent/runs/config-extraction-preplan.md
• Inline codex exec review --uncommitted saved under ignored .agent/runs/config-extraction-review.md: no discrete correctness regressions found

Notes

This is a behavior-preserving architecture extraction. It does not change YAML parser semantics, project config defaults, graph application order, or resource parsing behavior.