ci(release): placeholder release-prepare workflow so refactor's can be dispatched

[tejaskash]

Why this PR exists

The refactor branch has its own release process, merged in #2206: release-prepare.yml is dispatched by hand to open a release PR, and release-publish.yml publishes when that PR merges.

GitHub only lists a workflow in the Actions UI and the dispatch API if a file with that name exists on the default branch. release-prepare.yml exists only on refactor, so today it cannot be dispatched at all:

$ gh workflow run release-prepare.yml --ref refactor -f bump=major -f channel=rc
HTTP 404: workflow release-prepare.yml not found on the default branch

Main and refactor will stay separate for a while, and we need to cut 1.0.0-rc.1 from refactor now. This PR adds a placeholder release-prepare.yml to main so GitHub knows the name.

What it does

Nothing to main. When you dispatch release-prepare:

• with refactor selected in the branch picker, GitHub runs refactor's real release-prepare.yml with refactor's inputs (bump, channel). This is standard GitHub behavior: the selected branch's copy of the file runs.
• with main selected, this placeholder runs and fails with an error telling you to pick refactor. Main's package.json is never touched.

Main's own release process (release.yml, release-main-and-preview.yml) is unchanged.

When to remove it

When refactor lands on main, the real release-prepare.yml overwrites this file. Nothing else to clean up.

release-publish.yml needs no placeholder. It runs on pull_request: closed, which reads the workflow from the PR's own branch.

[agentcore-devx-automation]

AgentCore Harness Review

Verdict: Looks good

Small, self-contained CI change. Verified the mechanics:

• .github/workflows/release-prepare.yml filename and name: release-prepare match refactor's copy, so dispatching with --ref refactor correctly picks up refactor's workflow (including its bump/channel inputs) while --ref main runs this placeholder.
• The placeholder has no checkout, no write permissions, and no inputs — it just prints a ::error:: and exit 1, so nothing on main is touched if someone accidentally dispatches against main.
• No existing release-prepare.yml on main to conflict with, and release.yml / release-main-and-preview.yml are untouched as claimed.
• Telemetry/mocking guidance is N/A here (CI-only change, no runtime or test code).

The self-documenting comment at the top clearly explains why the file exists and when to remove it. Nothing blocking.

[agentcore-devx-automation]

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

[github-actions]

Package Tarball

aws-agentcore-0.28.1.tgz

How to install

gh release download pr-2226-tarball --repo aws/agentcore-cli --pattern "*.tgz" --dir /tmp/pr-tarball
npm install -g /tmp/pr-tarball/aws-agentcore-0.28.1.tgz

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	41.09%	15754 / 38332
🔵	Statements	40.35%	16802 / 41631
🔵	Functions	35.2%	2713 / 7706
🔵	Branches	34.26%	10537 / 30750

Generated in workflow #4450 for commit 1e3bf8b by the Vitest Coverage Report Action