Convert update-plugin-index to a composite action

[scotwells]

Why

The update-plugin-index reusable workflow (shipped in v1.17.0) can't actually do its job. Callers mint a GitHub App token to open the cross-repo catalog PR, but a reusable workflow runs as its own job, so the token has to cross a job boundary — and masked values are scrubbed to empty when passed through job outputs. In practice the catalog checkout failed with Input required and not supplied: token.

What this changes

Converts it from a reusable workflow to a composite action (update-plugin-index/action.yml) — the repo's first. A composite action runs inside the caller's job, so the token is minted and used in the same job and passed directly as the token input (no cross-job hop). The step logic and the generic archive→checksum mapping are unchanged; the only interface change is the cross-repo credential moving from a PLUGIN_INDEX_TOKEN secret to a token input.

• Adds update-plugin-index/action.yml.
• Removes .github/workflows/update-plugin-index.yaml.
• Updates docs/update-plugin-index/ and CLAUDE.md.

Callers now mint + call in one job:

steps:
  - uses: actions/create-github-app-token@v2
    id: app-token
    with: { app-id: ..., private-key: ..., owner: milo-os, repositories: cli-plugins }
  - uses: datum-cloud/actions/update-plugin-index@v1
    with:
      index-repo: milo-os/cli-plugins
      plugin-name: ipam
      version: ${{ github.event.release.tag_name }}
      token: ${{ steps.app-token.outputs.token }}

Verification

Validated end-to-end against the existing milo-os/ipam v0.2.0 release assets, opening a real PR on a scratch milo-os/cli-plugins branch (see comments).

Related work

• First consumer: Update cli-plugins catalog on release; fix goreleaser double-run milo-os/ipam#66
• Supersedes the reusable-workflow form shipped in v1.17.0 (Add update-plugin-index reusable workflow #84)
• datumctl plugin marketplace: Open the datumctl plugin marketplace to third-party catalogs datumctl#224