docs write --replace --markdown should support --tab (whole-tab re-render)

[sebsnyk]

What

gog docs write <docId> --tab=<tab> --file=<md> --markdown --replace is rejected with:

--markdown with --replace does not support --tab (Drive's markdown converter operates on entire documents)

The whole-document-only limitation is in Drive's markdown converter, not the Docs API. The Docs API itself can target tab ranges via documents.batchUpdate (insertText / deleteContentRange with the tab's content boundaries). gog could convert MD locally (or via the existing markdown path) and apply the result against the specific tab range, achieving whole-tab re-render without touching other tabs.

Why

Once a doc has multiple tabs, every meaningful update is per-tab. Today the options for updating a tab's contents are:

1. --append --markdown --tab — partially broken (docs write --markdown --append fails to insert tables: 'insert native table: table not found near index N' #592 tables, docs write --markdown --append emits literal bullets + per-line code paragraphs (no native list, no contiguous code block) #594 bullets + fenced code as literals).
2. find-replace --format=markdown --tab — surgical but doesn't help when the whole tab needs to change shape (re-ordered sections, replaced lists).
3. Manual paste — requires a human at the keyboard.

A clean whole-tab re-render is the natural primitive when the source-of-truth markdown changes; without it, agent-driven Doc workflows have to chain many surgical find-replaces and hope the existing tab text still matches.

Suggested behaviour

gog docs write <docId> --tab='Gold list' --file=gold.md --markdown --replace
# Replaces only the Gold list tab content with the markdown-rendered version of gold.md.
# Other tabs untouched.

Scope

• Whole-tab content replacement only (don't need section-level replace).
• Should produce the same paragraph/list/code/table fidelity as docs create --file --markdown (the converter path that already works correctly).
• Tab properties (title, parent, index) stay unchanged.

Context

Filed alongside #592 (tables in --append), #594 (bullets + code in --append), and #593 (pageless flag). Together these define the agent-side gaps for updating tabbed docs cleanly without paste.