ci: set a least-privilege default GITHUB_TOKEN scope (#1831)

CodeQL alert #64 (`actions/missing-workflow-permissions`, medium) on
`.github/workflows/main.yml`: no `permissions` block, so every job inherits the
repository's default GITHUB_TOKEN scope, which is broader than anything here
needs.

It matters more than the severity suggests because this is the workflow that
publishes to npm under OIDC — an over-permissioned token in a job that mints
publish credentials is worth tightening before 2.0.0 ships (#1818).

Adds a workflow-level `permissions: contents: read`. The `publish` and
`publish-github-container-registry` jobs already declare their own blocks
(`id-token: write` and friends) and are unaffected: job-level permissions
override the workflow-level default outright rather than merging with it. The
comment records that, since it means each publish job must keep listing every
scope it needs — including `contents: read` — and a future edit that trims one
"because it's inherited" would break the publish.

Surfaced by the v2 tree swap (#1817): `main` had never scanned this workflow
before. Pre-existing on `v2/main`, not introduced by that merge.


Claude-Session: https://claude.ai/code/session_01Txmv2qqv3yeKgRzoqXytzD

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/main.yml

@@ -8,6 +8,15 @@ on:
   release:
     types: [published]
 
+# Default least-privilege scope for GITHUB_TOKEN. Without this, jobs inherit the
+# repository's default token permissions, which are broader than any job here
+# needs (CodeQL `actions/missing-workflow-permissions`). The `publish` and
+# `publish-github-container-registry` jobs declare their own blocks below, which
+# override this one entirely rather than adding to it — so each publish job must
+# continue to list every scope it needs, including `contents: read`.
+permissions:
+  contents: read
+
 jobs:
   build:
     runs-on: ubuntu-latest