docs: prioritize idiomatic TypeSpec in lintdiff migrations

[Yuchao Yan (msyyc)]

Summary

Adopt the TypeSpec-first migration principle settled in #5361: native linter rules should validate idiomatic TypeSpec semantics, preserving Swagger parity only where it naturally maps to that contract. Exact executable Swagger behavior is not a requirement when it entails emitter/format/encoding simulation or handling constructs already invalidated by TypeSpec or Azure rules.

• Update the develop-lintdiff-rule skill's implementation boundary, evidence gate, fixture requirements, migration reporting, and review guidance. Added complexity must have tests demonstrating value for valid supported TypeSpec; otherwise omit it and document intentional parity gaps.
• Update ParametersSchemaAsTypeObject/migration.md to preserve historical corpus numbers and evidence while labeling exact-emission parity as historical, not the promotion acceptance criterion. State the promoted use-model-request-body contract: a plain model without an indexer, with absent/synthetic void and multipart exemptions.

Scope

Independent documentation-only PR against feature/lintdiff-migration-new. Contains exactly the skill and migration note; no production implementation, generated corpus data, or dependency changes. Markdown is excluded by the Chronus changedFiles configuration, so no release metadata is added.

The existing corpus evidence remains historical; no new corpus run or broad repository build/test was performed for this documentation change.