Define provider adapter boundaries

[Will-thom]

Summary

• add docs/provider-adapter-boundaries.md
• define package ownership for core analyzer, provider interfaces, and optional provider implementations
• document import rules, testability expectations, and local-first preservation
• link the boundary doc from integrations, provider context plan, and backlog

Validation

• git diff --check
• go test ./...

Closes #84

[github-actions]

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