Improve the agents chat and Agents section UX

[AllTerrainDeveloper]

Chat window:

• Conversation rows are two lines — agent name + the tail of the last message — with a timestamp. The title (first user message) moves to the tooltip; it repeated across every conversation that opened the same way.
• The header avatar and the sidebar row avatars open the agent in My WordPress.
• The viewer's avatar goes through <wpd-avatar> + the Gravatar probe, so a Gravatar-less user gets initials instead of the mystery-person silhouette.
• An object sent to an agent (drop or "Send to") renders as a clickable card that opens it in its own window, instead of the runner-facing boilerplate sentence. Persisted with the conversation.

Agents section:

• "Create agent" moves above the list (it used to be the last row of a scrolling column).
• The detail verbs get their own row under the name, and gain View contributions → the GitHub-style footprint.
• Role is a dropdown when editing an agent, not a free-text field.
• The Tools/Triggers loading spinner is big and centered.

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