Growth chat session for new users (#295229)

* Revert Growth agent isReadOnly infrastructure

Remove the isReadOnly extension point property and Growth-specific
infrastructure from the chat sessions API that was added in #294255.
This was scaffolding for a growth session approach via the extension
chatSessions API, which is being replaced with a core-side implementation.

Removed:
- isReadOnly on IChatSessionsExtensionPoint and its schema
- isReadOnly filtering in session target/delegation pickers
- contribution?.isReadOnly parameter from getAgentCanContinueIn
- Growth from getAgentSessionProvider (prevents cache persistence)
- Growth-specific and isReadOnly tests

Kept:
- AgentSessionProviders.Growth enum value (used by new implementation)
- ChatSessionStatus.NeedsInput in proposed API + ext host types
- Growth cases in name/icon/description switch statements

* growth notification for new chat users

* hygiene skill

* Update src/vs/workbench/contrib/chat/browser/chatSetup/chatSetupGrowthSession.ts

Co-authored-by: Copilot <[email protected]>

* Address PR review feedback

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Co-authored-by: Copilot <[email protected]>