feat: adopt changesets for versioning

[RatulMaharaj]

What

Sets up changesets for versioning, running entirely under Deno (npm:@changesets/cli, no Node needed locally).

Changesets only understands package.json, so a small shim bridges it to Deno:

• Private package.json stubs per package (plus a root with workspaces) exist solely for changesets to track and bump versions. JSR still publishes from deno.json.
• scripts/sync-versions.ts copies each bumped version into its deno.json.
• All three packages are in a fixed group, matching the existing lockstep releases.
• New tasks: deno task changeset (add a changeset) and deno task version (bump + sync + changelogs).
• version.yml runs changesets/action on pushes to main and opens a "Version Packages" PR; merging it is the cue to cut a GitHub release, which triggers the unchanged publish-jsr.yml.

Also adds CLAUDE.md: always use changesets; always read skills/writing/looped-docs.md before writing docs.

Testing

Smoke-tested end to end locally: a test patch changeset bumped all three packages in lockstep across both package.json and deno.json and generated changelogs (then reverted).

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[RatulMaharaj]

Closing without merging — we're staying on the simpler release flow instead.

Two things surfaced while trying this locally:

1. The package.json stubs flip Deno into node_modules resolution mode for the whole workspace, which breaks deno task test / ok / CI unless a node_modules dir is materialized (or nodeModulesDir: auto is set). The shim isn't inert — it changes how the runtime resolves every npm dep.
2. The machinery outgrows the problem. Three packages in a fixed group releasing in lockstep means changesets is managing exactly one version number — at the cost of a second version source of truth, a sync script, a bot-opened Version Packages PR, and stub files that exist only for the tool.

The flow that shipped v0.2.0 already covers the goals: versions bump in one commit, the release event publishes JSR + immutable image tags together, and a CI guard fails the release if the tag disagrees with VERSION. The one remaining pain — bumping four files by hand — is a five-line deno task bump script, not a versioning framework.

Worth salvaging from this branch separately: the CLAUDE.md (minus the changesets rule) and possibly changelog generation, which gh release create --generate-notes covers for now.

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