feat(docs): add insert-table primitive

[chrischall]

Closes #602.

Summary

Surfaces the Docs API `InsertTableRequest` as a standalone CLI subcommand that bypasses the markdown writer. The markdown path can render tables, but it has had several distinct rendering bugs (#592, #608, #609) and structural off-by-ones (#607) — for agent-generated table-shaped deliverables (itineraries, schedules, comparisons) a direct primitive is the cleanest backbone.

Usage

```
gog docs insert-table --rows N --cols M [--index N | --at-end] \
[--values-json [[...]]] [--tab=STRING]
```

Flag	Notes
`--rows`, `--cols`	Required. Must each be `>= 1`.
`--index` / `--at-end`	Omit either to insert at end-of-doc (same convention as `docs write --append` and the new `insert-page-break` command); `--at-end` is the explicit form. Mutually exclusive with `--index`.
`--values-json`	JSON 2D string array. Dimensions must match `--rows` x `--cols` exactly; omitted = all-empty cells.
`--tab`	Standard tab-by-title-or-id support.

Internals

• Reuses the existing `TableInserter.InsertNativeTable` so the structural insert, the post-insert `Get` to find cell indices, and the per-cell text inserts all go through the same path the markdown writer uses (and pick up any future improvements there).
• End-of-doc resolution uses `docsTargetEndIndexAndTabID` + `docsAppendIndex`, matching the new `insert-page-break` PR and the recently filed `gog docs insert: default --index to end-of-doc (or rename / require it explicit) #606` insert default.
• `parseTableValuesJSON` validates dimensions and rejects malformed input with a precise usage error.

Tests

• `TestParseTableValuesJSON_EmptyProducesAllEmptyMatrix` — empty input → all-empty rows x cols.
• `TestParseTableValuesJSON_PopulatedMatrixRoundTrip` — happy path.
• `TestParseTableValuesJSON_RowCountMismatch` — row count enforced.
• `TestParseTableValuesJSON_ColumnCountMismatch` — per-row column count enforced with the offending row index.
• `TestParseTableValuesJSON_InvalidJSONRejected` — surface the JSON parse error wrapped in a usage error.
• `TestDocsInsertTableCmd_FlagValidation` — table-driven coverage of every `Run()` validation branch (`--rows`/`--cols` < 1, `--index`+`--at-end` conflict, negative `--index`, mismatched `--values-json`).

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