Consolidate v6 changesets to reflect the final changelog (#4411)

## What

Consolidate the pending changesets on `graphiql-6` so they describe the
finished v6 release rather than the path we took to build it.

`graphiql-6` is unreleased and in changesets pre-mode with `pre.json`
`changesets: []` (**no alpha has consumed any changeset yet**), so this
pending set becomes the `6.0.0` changelog verbatim. It had grown to **62
entries** written as if each incremental step would be felt by a user
("add `TopBar` with a placeholder URL" → later "wire the real URL"; a
dozen separate `restyle-*` bullets). None of those intermediate states
ship.

This collapses the set to **15 changesets** that read as the actual
changelog.

## Approach

| Bucket | Changesets |
| --- | --- |
| **Redesign** (one entry, self-explanatory) | `v6-redesign`: tokens,
light/dark theming, layout chrome (top bar, activity rail, side panel,
status bar, flattened editor, vars/headers strip), restyled components,
Monaco themes, doc explorer + history rebuilds, unified syntax colors,
browserslist |
| **One per feature** | `transport-api`, `response-pane`, `settings`,
`collections`, `query-builder`, `operation-name-follows-cursor` |
| **New `@graphiql/react` primitives** (new public API, one per concern)
| `keycap-hint`, `method-pill`, `segmented-control`, `panel-header` |
| **Breaking changes** (each stands alone) | `remove-deprecated-hooks`,
`drop-legacy-client-alias`, `explorer-deprecation` |
| **Unrelated package, untouched** | `graphql-value-string-highlighting`
(`vscode-graphql-syntax`) |

Feature changesets are written as end state. E.g. the response pane
header simply shows real status/time/size, with no mention of the
placeholders it briefly had.

## Editorial calls worth a look

- Dropped `v6-alpha-line` ("Initial v6 alpha"); its `#4219` reference
and migration-guide pointer fold into `v6-redesign`, which is now the
`graphiql` major headline.
- `history-method-pill` folds into the redesign's history bullet; its
small additive `QueryStoreItem.operation` field on `@graphiql/toolkit`
is no longer called out separately (toolkit is majored by
`drop-legacy-client-alias` regardless, and the redesign narrative
shouldn't land in the toolkit changelog).
- `query-builder` left at `plugin-query-builder: patch`, preserving
existing semver intent, not editing it. Flag if you want it minor.

## Validation

`changeset status` yields an **identical** set of package bumps before
and after, verified against `origin/graphiql-6`:

- **major:** `graphiql`, `@graphiql/react`, `@graphiql/toolkit`,
`@graphiql/plugin-doc-explorer`, `@graphiql/plugin-history`,
`@graphiql/plugin-explorer`
- **minor:** `@graphiql/plugin-collections`
- **patch:** `@graphiql/plugin-query-builder`, `vscode-graphql-syntax`,
`@graphiql/plugin-code-exporter` (transitive)

No version resolves differently; this is purely changelog-narrative
editing.

Author: trevor-scheer