docs: document dependency import convention

[leopoldjoy]

Summary

• document that dependencies should be imported through explicit lib/<dependency>/... paths
• clarify that existing Foundry remappings are special cases rather than precedent

Documentation-only change; no tests required.

[cb-heimdall]

✅ Heimdall Review Status

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Reviews	✅ 1/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1