Docs: clarify agent tag format and registration schema

[santoshparastv]

Summary

The README shows agent registration with tag: {1, 3, 4} and mentions combinations like 1+4, but the exact tag format and parsing rules are unclear.

Proposal

• Define a canonical tag representation (e.g., array of ints, array of strings, or string expression)
• Clarify whether 1+4 means set union, a composite tag, or a routing hint
• Add 2-3 examples of valid registrations

Why

Clear rules reduce invalid agent registrations and improve agent selection consistency.