chore/support-catalogs-2

[gevalo1]

Closes 1707. Supersedes 1933

Note: AI-assisted (Claude Opus 5) while writing, human-reviewed and human-tested.

Problem

Catalogs let a workspace declare a version range once and reference it from many
packages through the catalog: protocol. Changesets doesn't understand the
protocol today, which shows up in three places:

1. catalog: is a protocol range, so a workspace package referenced through one
   is treated as invalid by getDependencyGraph, which prints "must depend on
   the current version of ...".
2. Releasing a package that a catalog points at leaves the catalog entry stale.
   Nothing updates it, so the workspace keeps installing the old version.
3. Updating a catalog entry marks nothing as changed. Editing the same range in
   a package's own package.json does mark that package as changed, because
   change detection is file based and the file lives inside the package. A
   catalog lives at the workspace root and belongs to no package in particular.

What this does

Adds @changesets/catalogs, which knows where each package manager keeps its
catalogs, how to resolve a catalog: reference, and how to write an updated
range back:

Package manager	Location
pnpm	catalog / catalogs in pnpm-workspace.yaml
Yarn	catalog / catalogs in .yarnrc.yml
Bun	catalog / catalogs in package.json, at the root or under workspaces

Both the default catalog (catalog:, also spelled catalog:default) and named
catalogs (catalog:react18) are supported.

On top of that:

• Resolution. Ranges are resolved wherever Changesets reads a dependency
  range — the dependency graph, dependent bumps, and changelog entries — so a
  catalog: reference behaves exactly as the range written out in full would.
• Versioning. Releasing a package that a catalog points at rewrites the
  catalog entry and keeps its range style (^1.0.0 becomes ^2.0.0). The
  packages referencing it keep saying catalog:. Entries that no package in the
  workspace references are left alone, matching what bun update does.
• Change detection. An updated catalog entry counts as a change to every
  package referencing it, for changeset add and changeset status. New
  detectCatalogChanges config option, default true, to opt out.

Notes for review

• The YAML and JSON writers edit in place using node ranges rather than
  re-serialising, so comments, quote style and ordering in pnpm-workspace.yaml
  survive untouched. Same approach as the existing editJson.
• Packages.catalogs is optional. Making it required would enforce it at the
  type level, but it breaks assembleReleasePlan(changesets, await getPackages(cwd), config)
  for external consumers, which is the usage documented in
  @changesets/get-dependents-graph's README. Entry points that build a graph or
  a release plan call a small withCatalogs helper instead.
• pack and publish need nothing extra — pnpm, Yarn and Bun each replace
  catalog: during their own pack step.

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The commits are split by layer, so each is reviewable on its own.