Manage agent permissions in a dedicated modal

Managing who can access an AI agent now lives in its own Manage Permissions modal, instead of being buried in the agent's Settings tab.

• When creating a new agent, a Manage Permissions button sits next to Save.
• When editing an existing agent, Manage Permissions is available from the More Actions menu.
• Permission changes only take effect after clicking Apply, so it's easier to review and discard edits before they go live.

The standalone Manage Access modal (accessed from the Agent Library and Conversation views) received the same consistency polish.