chore(phaseI): define composer dependency contract and document dependency policy

- Finalize composer.json with minimal public requirements and explicit suggests
- Keep optional drivers out of the public dependency contract
- Document dependency policy and adapter-to-driver matrix
- Clarify development vs public dependency responsibilities
- No runtime or source code changes

Author: megyptm

docs/07-dependencies.md

@@ -110,7 +110,7 @@ Each optional dependency is declared as:
   "predis/predis": "Required for RedisPredisAdapter",
   "mongodb/mongodb": "Required for MongoDatabaseAdapter"
 }
-````
+```
 
 This is **not optional documentation**.
 It is part of the dependency contract.