ci: fix fromJson('') crash that fails release job on non-release pushes (LAB-865)

[27Bslash6]

LAB-865: fix fromJson('') crash in release workflow assign step

The bug

GitHub Actions evaluates a step's env: templates when materializing the step, before the if: guard is consulted. On any push to main where release-please creates/updates no release PR, steps.release.outputs.pr is empty, so fromJson('') throws Error reading JToken from JsonReader and fails the whole release-please job — which skips publish (needs: release-please).

Evidence: runs 30205236363 (merge of #57), 29831146861, 29831120533, 29558428840 — all dead at this step. Last green: 29454532970 (v0.3.0, before the assign step existed).

Worst case: the push that merges a release PR also has an empty pr output, so release_created=true would be computed but the job dies first → tag/GitHub release and crates.io silently diverge. This would bite the LAB-764 v0.4.0 release with certainty.

The fix

Remove fromJson from the template position that can see an empty string. The step now selects the release PR by release-please's deterministic head branch:

release-please--branches--main--components--cachekit-core

Note: this is not the release-please--branches--main form — release-please v4 in manifest mode appends the component name. Verified against the actual release PRs #38 and #49, both on exactly this branch.

Both live paths survive

• Push with no release PR (outputs.pr empty): no template in the step parses JSON, so materialization succeeds; if: is falsy → step skipped → job green → publish gated only by release_created as designed.
• Push that creates/updates a release PR (outputs.pr set): if: truthy → gh pr edit <head-branch> resolves the open release PR (release-please maintains exactly one open PR on that branch) and adds the assignee; re-runs are idempotent (adding a present assignee is a no-op).

Scope

• Only the assign step changed (5 insertions, 2 deletions). Publish job, permissions, concurrency, triggers untouched.
• This is release-infra repair, not a check-gating change: this workflow publishes releases and is not a required PR status check.
• The LAB-791 Release-As: 0.3.1 footer was separately ineffective (buried mid-body by squash-merge; test: is a non-releasing type). Per the amended LAB-764 rollout, v0.3.1 is not being resurrected — the readers-first release is collapsed into v0.4.0. This PR fixes the pipeline only.

Docs gate

No docs surface: internal CI repair, no user-facing or documented behavior changed.

Closes LAB-865

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Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between 49a7017 and 4776d22.

📒 Files selected for processing (1)

• .github/workflows/release.yml

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch lab-865-release-fromjson-fix

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