ci(validate): skip changeset-check on release PRs

[lifebugz]

Summary

Surfaced by the release-flow verification in PR #23: the `changeset-check` job in `validate.yml` fails on release PRs because it expects to find a new changeset added by the branch, but release PRs (the ones opened by `changesets/action`) DELETE changesets — they have nothing to report.

This was masked until today because prior release PRs (#15, #21) either merged before branch protection existed or were admin-overridden. PR #24 is the first release PR that actually had its CI scrutinized.

Fix

Add a head-ref exclusion so `changeset-check` skips on PRs from `changeset-release/main` (the canonical branch name `changesets/action` uses):

```diff
changeset-check:

• if: github.event_name == 'pull_request'

• if: github.event_name == 'pull_request' && github.head_ref != 'changeset-release/main'
  ```

Why this is safe

• Skipped jobs satisfy required status checks in GitHub's branch protection (this has been the case since 2022).
• Release PRs are constructed mechanically by `changesets/action` from already-validated changesets; re-validating "is there a changeset" on them adds no signal.
• Human PRs still hit `changeset-check` — the rule is unchanged for the case it's meant to enforce.

Test plan

• Empty changeset present
• Post-merge: trigger PR chore(release): version packages #24 to re-check (close+reopen), confirm changeset-check is now reported as skipped, validate passes, PR mergeable through standard flow

Follow-up

After this PR merges, PR #24 needs a check re-trigger to pick up the new workflow. Close+reopen does it.