refactor(repo-graph): move helpers into module

[t3chn]

Summary

• Move repo graph id and path helper functions into a dedicated helpers module.
• Update repo graph submodules to import helper functions through the new module.
• Keep manifest parsing and ignored-path detection in the root module for separate follow-up slices.

Issue

• N/A

Test plan

• cargo check
• cargo fmt --check
• git diff --check
• scripts/run-deterministic-tests.sh