docs: prioritize idiomatic TypeSpec over emitter parity in lintdiff migrations

[Yuchao Yan (msyyc)]

Summary

Update only .github/skills/develop-lintdiff-rule/SKILL.md to codify the migration principle settled in PR #5361: prefer idiomatic TypeSpec validation and preserve Swagger parity only where it naturally maps to TypeSpec semantics.

Exact executable Swagger behavior is not the migration contract when reproducing it requires non-idiomatic checks, AutoRest schema formatting/encoding simulation, or handling constructs already invalidated by TypeSpec or Azure rules. The guidance now requires tests showing that added special cases affect valid supported TypeSpec; otherwise, omit the complexity and document the intentional parity gap.

Align coverage classification, the semantic completeness gate, fixture requirements, migration evidence, and review guidance with this priority. Corpus/lintdiff and emission-matrix evidence remain useful research tools, not mandates to reproduce emitter behavior. This follows Timothée Guérin's plain-model/indexer guidance and Catalina Peralta's confirmation of the idiomatic-TypeSpec-first principle.

Scope

Independent documentation-only PR based on and targeting feature/lintdiff-migration-new, not main. No production rules, fixtures, generated corpus data, dependency changes, or release metadata. Chronus excludes Markdown files from change detection.

Validation

• Prettier check limited to the changed skill document.
• git diff --check.
• No repository-wide formatting, build, or tests for this documentation-only change.