feat: make agent memory context limit configurable

[Purujeet-git]

Summary

• Adds a configurable memory_context_max_chars setting for each agent.
• Keeps the default memory context limit at 2000 characters for backward compatibility.
• Passes the configured limit through agent configuration into LLM context building.
• Adds an Alembic migration for the new agent setting.
• Adds tests covering the configurable memory context limit.

Fixes #810

Checklist

• Tested locally
• No unrelated changes included