release skill: link the Prepare Python Release PR to the release issue with addCloseIssueReferences #5335

Every PR is linked to its issue through the API (addCloseIssueReferences), not only by a Closes #N line, which on v2/main is a cross-reference only. The release skill already does this for the Version Packages PR (step 2b.1), but step 2c, Prepare Python Release, does not: the workflow writes Closes #N as the body's first line and nothing links the PR. scripts/prepare-python-release.mjs and prepare-python-release.yml make no call either.

Expected: the Prepare Python Release PR is linked to the release issue through the API and read back (closingIssuesReferences lists #N), the same as every other PR into v2/main.

Fix: in .claude/skills/release/SKILL.md step 2c, add the link-and-read-back step from /pr-flow step 6 for the workflow-opened PR and for the by-hand fallback. RELEASING.md should say it next to the Version Packages sentence. Optionally have the workflow make the call itself, if its token can; otherwise the skill step is the rule.

Out of scope: the milestone merge PR into main, which deliberately carries a non-closing Part of #N and no closing link, since on the default branch a link would close the release issue at merge.