fix(desktop): improve model picker message and runtime dropdown clarity

[wpfleger96]

The model picker dropdown in the agent list showed "This agent uses the runtime's default model" even when a custom model was explicitly set on the agent (e.g., kgoose-gpt-5-5 via Databricks). This happened because the message was gated solely on whether the ACP runtime supports model switching (supportsSwitching), without checking whether agent.model was already configured. The dropdown now shows the configured model name with a "switching not supported" note when the runtime doesn't enumerate models via ACP, and falls back to the original default-model message only when no model is set.

The "Add agents" channel dialog's Runtime dropdown had no indication that personas with a preferred runtime would silently override the dropdown selection. Added conditional helper text: when any selected persona has a provider set, the text explains that those personas will use their own runtime; otherwise it shows a generic "default runtime" note.

• ModelPicker.tsx — split the !supportsSwitching branch to check agent.model before choosing the message
• AddChannelBotDialog.tsx — added <p> helper text after the Runtime <DropdownMenu>, conditional on selectedPersonas.some(p => p.provider)

[wpfleger96]

Old behavior: model picker on right side says "This agent uses the runtime's default model" even though that isn't true (has custom goose-claude-opus-4-6 specified)

and the picker is broken anyway - the menu never drops down, and I can't switch models

New behavior:

• for harnesses that don't support model switching (e.g. sprout-agent), the picker now reflects the custom already-set model and shows help text about switching models

• for harnesses that do support model switching (e.g. goose), the picker now gives me a list of available models to switch to