[Infra] .all-contributorsrc only tracks README.md, so contributor badges never sync to backend/frontend/contracts READMEs

[grantfox-oss]

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Why this matters

.all-contributorsrc at the repo root has "files": ["README.md"], meaning the all-contributors bot only ever updates the root README table. Contributors who work primarily in backend/, frontend/, or contracts/ and whose sub-project READMEs mention contributors get no automated sync, and the contributor table can drift.

Acceptance criteria

• Confirm which files should list contributors (root README plus any sub-package READMEs that already have a contributors section)
• Update .all-contributorsrc files array accordingly
• Document in CONTRIBUTING.md how to trigger @all-contributors please add @user for code

Files to touch

• .all-contributorsrc
• CONTRIBUTING.md

Out of scope

• Adding new contributors manually
• Changing the all-contributors bot workflow itself