feat(docs): add insert-page-break primitive

[chrischall]

Closes #604.

Summary

Surfaces the Docs API `InsertPageBreakRequest` as a standalone CLI subcommand. Markdown has no native page-break construct that the markdown writer can translate, and `docs format` only restyles existing content — so this is the first structural-element insert primitive on the docs side.

Usage

```
gog docs insert-page-break [--index N | --at-end] [--tab=STRING]
```

Aliases: `page-break`, `pb`.

Form	Behaviour
insert-page-break <id>	Insert at end-of-doc (default; uses docsTargetEndIndexAndTabID + docsAppendIndex, same helpers as docs write --append)
insert-page-break <id> --at-end	Explicit end-of-doc
insert-page-break <id> --index N	Insert at body index N (>=1)
… --tab=Name	Works with both forms; tab resolved by title or ID

--at-end and --index are mutually exclusive. --index < 1 is rejected (index 0 is reserved, matching the existing docs insert guard).

Tests

• TestDocsInsertPageBreakCmd_ExplicitIndex — verifies single InsertPageBreakRequest with the given index and no extra GET.
• TestDocsInsertPageBreakCmd_DefaultsToEndOfDoc — omitted flags → one GET + insert at endIndex - 1.
• TestDocsInsertPageBreakCmd_AtEndFlag — explicit --at-end behaves identically.
• TestDocsInsertPageBreakCmd_AtEndAndIndexRejected — mutual exclusion guard.
• TestDocsInsertPageBreakCmd_NegativeIndexRejected — --index=-1 rejected with the same message as docs insert.
• TestDocsInsertPageBreakCmd_WithTab — tabs resolution + Location.TabId populated.

go test ./internal/cmd/ -count=1 → ok 23.5s.

Notes

This PR contains 2 commits — the feature, plus a follow-up gofmt re-alignment of the DocsCmd struct tags after adding the new field. Happy to squash on merge.