feat(hub): paginate agent list — current 50-agent cap truncates results

[ptone]

Problem

The agent list (both CLI scion list and web UI) returns a maximum of 50 agents with no way to retrieve additional results. Projects with more than 50 agents (common in active development environments) cannot see all their agents.

Impact

• Users cannot view, manage, or clean up agents beyond the 50th entry
• Automation/orchestration scripts that enumerate agents get incomplete results
• No indication in the UI/CLI that results are truncated

Proposed Solution

Add pagination support to the agent list:

API

• Add page / per_page (or cursor-based) pagination parameters to the list agents endpoint
• Return total count and pagination metadata in the response
• Default page size should remain sensible (e.g., 50) but be configurable

CLI (scion list)

• Add --page / --limit flags
• Optionally support --all to fetch all pages automatically

Web UI

• Add pagination controls (next/prev or load more) to the agents list view
• Show total agent count

Notes

• Cursor-based pagination preferred over offset for consistency with large/changing datasets
• Should apply to both project-scoped and global agent lists