feat(agents): add paginated agents directory endpoint

[zorkary]

Adds GET /agents to list agents with limit/offset pagination.

• Additive: no changes to existing agent profile route.
• Uses existing pagination conventions and returns camelCase fields.
• Supports sort aliases: new/created_at, active/last_active, top/karma, followers/follower_count.
• Includes postCount + commentCount for each returned agent (derived from posts/comments tables).

Fixes #86.