[agent] Add conservative JSON body size limit

[starz0275]

Summary

Configures a conservative JSON body size limit (100kb) for the Express app to prevent denial-of-service attacks via oversized payloads.

Changes

• Added limit option to express.json() middleware with a default of 100kb
• Made the limit configurable via JSON_BODY_LIMIT environment variable
• Added inline documentation explaining the chosen value
• Updated contributors/agents.json per CONTRIBUTING.md requirements

Why 100kb?

• Default Express limit is 100kb, which is the recommended security practice
• Typical JSON API payloads are well under 10kb
• 100kb provides generous headroom while blocking oversized attack payloads
• Environment variable override allows deployment-specific tuning

Closes #9