fix(chat): start without a screen for the tab status - #16

What

v1.10.0 panics as soon as the chat starts:

panic: runtime error: invalid memory address or nil pointer dereference
ui.(*Screen).do(0x0, …)            internal/ui/screen.go:169
ui.(*Screen).TabStatus(...)        internal/ui/terminal.go:64
cmd.(*chatSession).tabStatus(...)  cmd/chat.go:290
cmd.(*chatSession).reset(...)      cmd/chat.go:561
cmd.runChat(...)                   cmd/chat.go:148

runChat calls reset() before it creates the screen, and reset() now clears the tab status through s.scr, which is still nil at that point. One-shot questions (docsgpt-cli "…") are unaffected.

Fix

tabStatus returns when there is no screen yet: nothing can be shown on the tab before then.

Testing

• New TestResetBeforeScreen: panics without the fix, passes with it.
• gofmt -l ., go vet ./..., go test ./...
• Started the binary in a pseudo-terminal with a throwaway HOME: the v1.10.0 code panics, this branch brings up the chat screen.