Bump herald to 0.1.2.0 and herald-release action to 0.0.2.0

[carbolymer]

Context

This updates the herald changelog tooling used by this repository:

• release.yml: the herald-release action is bumped from herald-release-0.0.1.0 to herald-release-0.0.2.0.
• Both herald-validate and herald-release now pass an explicit herald-ref: github:input-output-hk/cardano-dev/herald-0.1.2.0.
  The actions fetch the herald binary at runtime from their herald-ref input, whose built-in default is stale at herald-0.1.1.0, so bumping the action tag alone would not update the binary.
  The override also matters for correctness: the herald-release-0.0.2.0 script was rewritten for per-project fragment directories, which only exist in herald 0.1.2.0.
• flake.lock: the cardano-dev input is updated to the latest main, so the dev shell provides herald 0.1.2.0 as well.
  This also re-locks cardano-dev's transitive haskellNix subtree, because cardano-dev switched its haskellNix input from a fork back to input-output-hk/haskell.nix between the two pins, and keeping the old transitive entries would fail pure evaluation.

How to trust this PR

• The flake.lock diff touches only the cardano-dev node and nodes scoped under it (cardano-dev/haskellNix/...); none of cardano-api's own inputs moved.
• The changelog check on this PR already runs the updated check-pr-changelog.yml, so a green "Check changelog fragments" status shows herald 0.1.2.0 validating this repository.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff
• Changelog fragment added in .changes/

[Copilot]

Pull request overview

Updates repository release/changelog tooling to use herald 0.1.2.0, aligning CI actions and the dev shell with the newer herald version via the cardano-dev flake input.

Changes:

• Bump cardano-dev (and related Nix inputs) in flake.lock.
• Update the release workflow to use herald-release-0.0.2.0 and explicitly pin herald-ref to herald-0.1.2.0.
• Update the PR changelog check workflow to pass herald-ref so it no longer relies on the 0.1.1.0 default.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 1 comment.

File	Description
flake.lock	Updates pinned Nix inputs, including cardano-dev, to pull in herald 0.1.2.0 via the dev environment.
.github/workflows/release.yml	Bumps herald-release action and pins herald-ref to herald-0.1.2.0 for releases.
.github/workflows/check-pr-changelog.yml	Adds herald-ref input to herald-validate to ensure validation runs with herald 0.1.2.0.
.changes/20260818_cardano-api_bump_herald_tooling.yml	Adds a changelog fragment documenting the herald tooling bump.

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