Add managed agent creation and reliable mentions

[spytensor]

What changed

• add a single runtime-owned create_agent tool that writes through the existing Agent definition service
• keep Agent paths, schema validation, cache invalidation, and registration inside the runtime
• make @Agent suggestions survive a delayed roster response
• send valid manually typed Agent mentions as structured mentions

Root cause

Agent creation was exposed through the management API but not through the model tool surface, so models guessed filesystem locations. Mention recognition also discarded the active query while the asynchronous Agent roster was still empty, and only popup-selected names were sent structurally.

Verification

• targeted Agent authoring test passed
• InputBar tests passed (30/30)
• backend and UI type checks passed
• backend build passed
• public export privacy check passed