add unsafe Effect assertion diagnostic

[mattiamanzati]

Summary

• port the upstream unsafeEffectTypeAssertion diagnostic into the Go implementation, including a quick-fix that removes unsafe assertions narrowing Effect error or requirements channels
• map the per-channel explanation to related diagnostics in Go, instead of baking multiline detail into the top-level message, while preserving the underlying behavior
• add v3/v4 fixtures, preview coverage, baselines, metadata updates, README updates, and a changeset for the new diagnostic

Validation

• pnpm setup-repo
• go test ./internal/effecttest -run 'TestEffectDiagnostics/unsafeEffectTypeAssertion|TestEffectV3Diagnostics/unsafeEffectTypeAssertion|TestEffectQuickFixes/unsafeEffectTypeAssertion|TestEffectV3QuickFixes/unsafeEffectTypeAssertion' -count=1
• UPDATE_METADATA_JSON=1 go test ./internal/rules -run TestUpdateMetadataJSON -count=1
• UPDATE_README=1 go test ./internal/rules -run TestUpdateReadme -count=1
• go test ./internal/rules -run 'TestMetadataJSON|TestReadmeTable' -count=1
• pnpm lint
• pnpm check
• pnpm test

Notes

• The upstream TypeScript implementation formats channel details into the main diagnostic string. This Go port emits those details as related diagnostics because this repo version supports child diagnostics cleanly.