feat(docs): expose heading styles via docs format

[chrischall]

Closes #605.

Summary

`docs format` covered text attributes and a couple of paragraph attributes (alignment, line spacing) but had no way to set a paragraph's named style — the Docs API field that drives the outline view / navigation pane and is the difference between a flat doc and a structured one. The markdown writer can produce HEADING_N on fresh content via leading `#`s, but offered no way to restyle existing paragraphs.

New flags

• `--heading-level N` (1..6): shortcut for `HEADING_N`.
• `--named-style NAME`: full enum — `NORMAL_TEXT`, `TITLE`, `SUBTITLE`, `HEADING_1`..`HEADING_6` (case-insensitive).

Mutually exclusive. Either path sets `paragraphStyle.namedStyleType` on the existing `UpdateParagraphStyle` request, so they compose cleanly with `--alignment` and `--line-spacing` without an extra round-trip.

```
gog docs format --match=Title --heading-level=1
gog docs format --match=Title --named-style=HEADING_1
gog docs format --match="Day-by-Day" --heading-level=2 --alignment=center
```

Tests

• `TestDocsFormatFlags_HeadingLevelEmitsNamedStyleType` — H1/H2/H6 → correct enum + Fields includes `namedStyleType`.
• `TestDocsFormatFlags_NamedStyleAcceptsAllValidEnums` — round-trips every legal value.
• `TestDocsFormatFlags_NamedStyleIsCaseInsensitive` — `heading_3` → `HEADING_3`.
• `TestDocsFormatFlags_HeadingLevelAndNamedStyleMutuallyExclusive` — combining the two errors out.
• `TestDocsFormatFlags_HeadingLevelOutOfRangeRejected` — -1, 7, 99 all rejected with a precise message.
• `TestDocsFormatFlags_UnknownNamedStyleRejected` — `BANNER` → enum error.
• `TestDocsFormatFlags_HeadingComposesWithAlignment` — single paragraph request carries both `alignment` and `namedStyleType` Fields, with `TabId` preserved.
• `TestDocsFormatFlags_AnyDetectsHeadingFlags` — `any()` sees the new fields so the "no formatting flags provided" guard stays correct.

`go test ./internal/cmd/ -count=1` → ok 23.6s.