ci(pr-flow): add PR base guard — block stacked PRs

[0xmariowu]

Why

Sister project lazie had stacked PRs (#108/#109/#110) desync under squash + auto-merge today. Adding the same guard repo-wide.

Topology-aware auto-merge wouldn't fix it: squash merge produces new SHAs that downstream PR base refs don't reflect, so the chain breaks even with correct ordering.

What

.github/workflows/assert-base-is-default-branch.yml — any PR whose base.ref != repository.default_branch fails. Branch from main, merge serially.

Test plan

• This PR's base = main → check passes
• After merge, intentional stacked PR is rejected

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 9d171133-2b6e-4c00-a7b1-d1f552acd3fd

📥 Commits

Reviewing files that changed from the base of the PR and between 1ada0b9 and fbb6475.

📒 Files selected for processing (1)

• .github/workflows/assert-base-is-default-branch.yml

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[github-actions]

@copilot please address the blocking signals on this PR and push a fix commit.

Trigger: ci-failure
Details: Workflow "Semgrep" failed on run 25420414188

Follow the PR's .github/copilot-instructions.md hard rules. Do not suppress lints, skip tests, or weaken CI.

[Copilot]

Pull request overview

Adds a lightweight GitHub Actions guardrail to prevent stacked PRs (PRs targeting another PR branch), avoiding desync scenarios under squash merges + auto-merge.

Changes:

• Introduces a new workflow that fails any pull_request whose base branch is not the repository default branch.
• Emits a clear failure annotation instructing contributors to branch from the default branch and merge serially.