fs: extract session setup, remove dead code and adapter
- Extract SetupToolServer, BuildPromptLayers, LoadAutoLoadTools into setup.go
- Deduplicate session infrastructure setup between CreateAgent and restoreSession
- Delete adapter.go, move HandlerCtx and type aliases into spec.go
- Delete session.go (only contained unused Get/All functions)
- Remove unused: GlobalRootState, PostEvent, LookupByUname, AddSession,
CreateAgentFromRoot, KillFromRoot, RenameFromRoot, Session.Interrupt,
Session.SetDisallowTools, Session.RunnableID
- Replace RunnableID() calls with ID()
- Remove comment banners
Net: -131 lines, better DRY compliance
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