[BUG] Downstream smoke-test CHANGELOG diverges from workspace scaffold after release cycle

[c-vigo]

Description

The smoke-test dispatch workflow (repository-dispatch.yml) mismanages CHANGELOG.md in the downstream repo (vig-os/devcontainer-smoke-test). After a full release cycle, main's CHANGELOG.md does not match the workspace scaffold shipped with the image (assets/workspace/CHANGELOG.md).

Two problems:

1. Wrong source file: The "Sync upstream CHANGELOG onto release branch" step downloads vig-os/devcontainer/${TAG}/CHANGELOG.md (the upstream project changelog with real version entries) instead of assets/workspace/CHANGELOG.md (the empty scaffold). This overwrites the release branch with content that has no relation to the downstream repo.

2. No post-release reset: After prepare-release and release workflows process their fake ## Unreleased / ## [X.Y.Z] - TBD lifecycle, nothing resets CHANGELOG.md on main back to the scaffold. The fake changelog entries accumulate on main across release cycles.

Steps to Reproduce

1. Dispatch smoke-test-trigger with an RC tag (e.g., 0.3.1-rc9)
2. Observe the deploy step creates a stub CHANGELOG.md with "Deploy devcontainer 0.3.1-rc9"
3. prepare-release freezes it to ## [0.3.1] - TBD
4. The sync step overwrites with the upstream project's CHANGELOG (wrong file)
5. After the release PR merges, main's CHANGELOG.md contains upstream version entries that don't belong in the downstream repo

Expected Behavior

After the full release cycle completes, CHANGELOG.md on main in the downstream repo should be identical to the workspace scaffold (assets/workspace/CHANGELOG.md) -- just ## Unreleased with empty category headers.

Actual Behavior

main's CHANGELOG.md contains either:

• The upstream project's full changelog (if the sync step succeeded), or
• Leftover fake entries from prepare-release (e.g., ## [0.3.1] - TBD with "Deploy devcontainer 0.3.1-rc9")

Neither matches the scaffold. Current state of main in the downstream repo is an empty scaffold only because no release has fully completed yet.

Environment

• Template: assets/smoke-test/.github/workflows/repository-dispatch.yml (lines 504-537)
• Scaffold: assets/workspace/CHANGELOG.md
• Downstream repo: vig-os/devcontainer-smoke-test

Additional Context

The CHANGELOG sync step is also a source of merge conflicts between the release branch and main, contributing to #402. The prepare-release and release-core workflows require a valid ## [X.Y.Z] - TBD entry to function, so the fake CHANGELOG cannot simply be replaced before those workflows run.

Changelog Category

Fixed

• TDD compliance (see .cursor/rules/tdd.mdc)