Branch health guardrail surfaced as a PR check for all stacks

Today it is too easy for teams to keep building on top of a broken tracked branch without realizing it.
Concrete issues

A proposed run on a PR can pass, the PR gets merged, but the tracked run on the default branch fails only at apply time. So from the developer perspective the change looked safe.
The main signal that something is wrong lives in Spacelift UI and Slack notifications. Developers often ignore Slack because it is already noisy and outside their normal dev workflow.
Locking stacks is not a good fit here. It can block teams when the person who caused the issue is offline, and it adds more friction and social overhead instead of clarity.

As a result

Developers keep opening and merging PRs on top of an unhealthy tracked branch.
Platform teams spend time chasing people and explaining why their “green” PR actually introduced a problem.
This increases cognitive load.

I want to see a clear signal in my PR when the tracked branch for that stack is unhealthy
so that I know it is not safe to merge and I can coordinate fixing main first.
As a platform engineer I want branch health to be enforced as a PR check, not just a Slack notification
so that I do not have to rely on people watching Slack or learning Spacelift UI to avoid breaking workflows.
Desired behavior

Branch health state per stack

Spacelift maintains a simple health state for each stack’s tracked branch, for example healthy or unhealthy.
This state reflects the status of the latest tracked runs on the tracked branch, including failed applies that passed plan.

PR health check on every proposed run

For every proposed run triggered by a PR that touches a given stack, Spacelift evaluates the stack’s branch health.

If the tracked branch is unhealthy, Spacelift

fails a dedicated “Branch health” check in the VCS

optionally posts a comment on the PR explaining that main is unhealthy, with a link to the failing tracked run and its logs.

Scope

The health check applies to all future PRs affecting that stack from the moment the tracked branch becomes unhealthy.
It is not limited to the original PR that introduced the problem. Anyone trying to build on top of a broken main gets the same warning.