Support optional local code retrievers

[alphali]

Summary

• let the Doable answering Skill use already-installed local code retrievers for cross-module candidate discovery
• require verification against current original source before a candidate becomes remotely submitted evidence
• remain provider-neutral, keep retriever artifacts local, and fall back to direct repository search
• release the plugin contract as version 0.2.2

Verification

• npm test
• all three Skill quick validators
• plugin structure validator
• git diff --check

This PR does not install or depend on Graft and does not change the Doable API contract.