[Cosmos] Add .pragentrules for PR review automation

[jeet1995]

What

Adds sdk/cosmos/.pragentrules — review rules consumed by the PR Deep Reviewer agent as mandatory constraints. These encode Cosmos-specific knowledge the agent cannot infer on its own.

Why

PR #47566 merged a new public enum (QuantizerType) and public methods on CosmosVectorIndexSpec without @Beta annotation, even though vector search is a preview feature. No existing automation caught this — not CheckStyle, not Revapi, not the PR review agent.

These rules close that gap. Modeled after the .NET Cosmos SDK precedent.

Rules Added

Category	Rule	Why Not Redundant with Agent
Preview API gating	Check if new public APIs need @Beta — asks if service feature is GA	Agent cannot know Cosmos feature GA status
Public enum extensibility	Scrutinize new Java enums for closed-set risk	Agent does not check Java enum extensibility patterns
Azure SDK guidelines	Verify central Java design conventions for new client surface only	Agent does not reference central guidelines
Java 8 compatibility	No Java 9+ APIs/features in production code	Agent does not enforce language baseline
Changelog (suppress)	Do NOT flag missing per-PR entries	Prevents false positive — Java Cosmos batches at release
API contracts (suppress)	Do NOT ask for contract files	Prevents false positive — uses APIView/Revapi in CI
CheckStyle (suppress)	Do NOT suggest disabling lint rules	Prevents bad advice
Multi-region emphasis	Extra scrutiny on failover/routing for SW vs MW	Domain-specific critical area
Retry emphasis	Extra scrutiny on retry/timeout/exception-mapping	Domain-specific critical area

Testing

Ran pragentrules-resolver against PR #47566 changed files:

• QuantizerType.java, CosmosVectorIndexSpec.java, CHANGELOG.md, VectorIndexTest.java
• All 4 files correctly resolve to the 9 rules from sdk/cosmos/.pragentrules

Safety

• .pragentrules are consumed only by the PR Deep Reviewer agent — zero CI impact
• No code changes — this is a review-time instruction file only

[jeet1995]

Closing — moving .pragentrules to the cosmos-sdk-copilot-toolkit repo instead of placing it in each SDK repo. The PR Deep Reviewer agent will load rules from the toolkit as a fallback.