fix(ci): avoid formatting lock file

[Hweinstock]

Problem

See #1488, formatter is running on generated file causing dependabot PRs to fail CI. Since the file is generated, this check will always fail on updating, causing noise in CI and pre-commit.

Solution

• add shrinkwrap to prettier ignore file.

Testing

# regenerated the file
npm shrinkwrap
npm run format:check 
# I see the failure here. 
# update the .prettierignore
npm run format:check
# now it passes

[agentcore-devx-automation]

Claude Security Review: no high-confidence findings. (run)

[github-actions]

Package Tarball

aws-agentcore-0.18.0.tgz

How to install

gh release download pr-1489-tarball --repo aws/agentcore-cli --pattern "*.tgz" --dir /tmp/pr-tarball
npm install -g /tmp/pr-tarball/aws-agentcore-0.18.0.tgz

[agentcore-cli-automation]

Looks good to merge. Single-line fix to .prettierignore correctly addresses #1488 — npm-shrinkwrap.json is a generated file and shouldn't be subject to formatting. Verified the testing steps in the PR description make sense, and there are no related references to shrinkwrap formatting elsewhere in the repo (CI scripts, bundle script, etc.) that this could affect.