fix(docs): default docs insert --index to end-of-doc

[chrischall]

Closes #606.

Summary

Iterative gog docs insert calls without an explicit --index previously stacked at position 1, producing reversed output (each new insert pushed earlier inserts further right). The help even called this out — "Character index to insert at (1 = beginning)" — but it's not what anyone reading insert as the editor-style "add at the end" default expects, and it silently produces broken docs in the dominant agent use case (build a long doc in chunks across many tool calls).

Picks option (2) from the issue: when --index is omitted, resolve end-of-doc via the existing docsTargetEndIndexAndTabID / docsAppendIndex helpers (same path docs write --append uses) and insert there. Explicit --index N still works exactly as before and skips the extra GET.

Change

• DocsInsertCmd.Index is now *int64 instead of int64 with default:"1" — so we can distinguish "omitted" from "set to 1".
• Run() branches on c.Index == nil:
  • omitted → GET the doc, derive docsAppendIndex(endIndex), also resolves the tab in the same call
  • explicit → validate >= 1, use it directly, no GET
• Dry-run payload reports "atIndex": "end" when omitted, the explicit int otherwise. The text/JSON success output always reports the resolved integer index.
• Help text updated: Defaults to end-of-doc when omitted.

Tests

• TestDocsInsertCmd_DefaultsToEndOfDoc — asserts the GET runs once and the resulting InsertText.Location.Index matches endIndex - 1.
• TestDocsInsertCmd_ExplicitIndexSkipsGet — asserts no GET when --index is supplied.
• TestDocsInsertCmd_ExplicitIndexBelowOneRejected — preserves the existing >= 1 guard.

Existing TestDocsEditingCommands_WithTab (which exercises --index 5 --tab Second) still passes.

go test ./internal/cmd/ -count=1 → ok 23.8s.

Compatibility note

This changes default behaviour for callers that previously relied on the implicit --index=1. Per the issue's analysis, those callers were getting reversed output rather than something useful; the new default matches docs write --append and what insert means in any text editor. Anyone explicitly passing --index=1 is unaffected.