feat(tests): merge each change into a simulated fork in CI (#1924)

[braboj]

Closes #1924.

What

tests/run_fork_sync.py checks ADR-049's claim that a fork syncs with git merge upstream/main. It makes a fork from the merge base of HEAD and origin/main. The fork appends a stack (a renamed copy of the fixture) and its manifest entry, and adds a line to its own README, CLAUDE.md and CHANGELOG. The runner then merges in three scenarios:

ID	Upstream side	Owed
FS-01	Planted: appends its own stack at the manifest's end	Conflict in templates/manifest.yaml only, caught
FS-02	Planted: edits the README under its title	Conflict in README.md only, kept as the fork's
FS-03	HEAD (the PR's merge commit in CI), or a planted base: edit when HEAD adds nothing	No conflict outside fork-owned files; smoke passes in the merged tree with SAIT_UPSTREAM_REF at the upstream side

The upstream change is the PR's own diff rather than a fixed synthetic edit, so every PR is checked for whether a fork can take it. Each simulated commit asserts that it changed exactly the files it claims to.

Spec SAIT-INT-MNF-06-001A, with rows in INDEX and CODIFICATION. There is a new step in smoke.yml, and the runner is listed in CLAUDE.md and PLAYBOOK.

Control

An upstream commit that gives core/quality.md the section ID fork-sync-stack, committed in a scratch worktree (the landing count went from 0 to 1):

• upstream run_smoke.py SYS-02: PASS
• run_fork_sync.py: FS-03 FAIL, with nested smoke 1 failed (SYS-02)

Checks run locally (py 3.12)

• smoke: 35/35 PASS
• fork sync: 3/3 PASS. On this branch, nested smoke passed 35/35
• sync.py --check and resolve.py --check: clean
• conformance: 8 failed locally, the same 8 as origin/main in a pinned worktree (environment-dependent). The one reading, core/git.md:1898, flags that this diff touches .github/workflows/smoke.yml, an off-limits path: it adds the CI step on purpose.

No CHANGELOG entry: the change is to this repository's own tests and CI, and no consuming project would notice it.

🤖 Generated with Claude Code