The Add to My plugins button looked like nothing happened: the action
succeeded (the plugin showed up on the Plugins page) but the originating
panel only ever showed a transient "Sending..." state. Two root causes:
1. ProjectView toggles `hiddenPluginActionPaths` during the install action,
which unmounts `PluginActionPanel` while the API call is in flight.
The panel's own `noticeByFolder` state went with the unmount, so the
`setNoticeByFolder(...)` call after `await onRequestPluginFolderAgentAction`
landed on a dead fiber and the success notice never rendered.
2. The `PluginInstallOutcome` contract leaves `message` optional. When
it is absent the surface had no fallback affordance — the button
silently reverted to "Add to My plugins" with nothing else changed.
Lift `busyKey` / `noticeByFolder` / `runPluginAction` from `PluginActionPanel`
up into `AssistantMessage` so the state survives the unmount cycle. Render
notices inside the panel for visible folders and add an orphan-notice slot
that covers folders the parent has hidden in the meantime. Default to an
"Added to My plugins." notice when the install resolves without a message.
Publish / contribute paths still rely on outcome messages they always
emit, so they're unaffected.
Fixes#2876
