Add partition colocation rule

[avinashkamat48]

Summary

• add a partition colocation best-practice rule
• show why related items should share a partition key for transactional batch and single-partition reads

Verification

• git diff --check

Closes #171

[TheovanKraay]

Thanks for this, the colocation concept is valid, but it's already covered by the existing model-type-discriminator.md rule, which describes co-locating related entity types that share a partition key and transactional boundary. partition-query-patterns.md also covers the access pattern alignment side. That said, your PR adds value by explicitly calling out TransactionalBatch usage, which the existing rule doesn't mention. Could you enhance model-type-discriminator to incorporate that angle instead of adding a new overlapping rule? Your order/order-line example would work well there, just add a type discriminator field to stay consistent.

[TheovanKraay]

Just a follow-up: we recently merged #204, which added topic-specific skills alongside the monolith. We're in a transitional phase where both coexist while we evaluate whether agent routing can handle the split on its own. Until that's resolved, new rules need to live in both places.

Since this is a partition- prefixed rule, please also do the following when addressing the other review feedback:

1. Copy skills/cosmosdb-best-practices/rules/partition-colocation.md to skills/cosmosdb-partition-key/rules/partition-colocation.md
2. Run npm run build to regenerate AGENTS.md for both skills
3. Commit the changes

[jaydestro]

@avinashkamat48 there are some ongoing changes being evaluated to the structure that could require this to be modified. you'll definetely get notice when it's time to make any changes to avoid merge conflicts.

[TheovanKraay]

Thanks for this, the colocation concept is valid, but it's already covered by the existing model-type-discriminator.md rule, which describes co-locating related entity types that share a partition key and transactional boundary. partition-query-patterns.md also covers the access pattern alignment side. That said, your PR adds value by explicitly calling out TransactionalBatch usage, which the existing rule doesn't mention. Could you enhance model-type-discriminator to incorporate that angle instead of adding a new overlapping rule? Your order/order-line example would work well there, just add a type discriminator field to stay consistent.

@avinashkamat48 FYI, we've backed out skill splitting so instructions on separate note not required. However, this does have overlap with existing rules. As we're trying to be as lean as possible and avoid duplication, please address above.