.github: add automerge script and workflow for upstream sync

[lumachad]

Summary

• Make the automerge script fully generic: repo slug, target branch, and upstream branch are read from required repository variables (AUTOMERGE_TARGET_BRANCH, AUTOMERGE_UPSTREAM_BRANCH) instead of being hardcoded. The script exits with an error if any are unset.
• Push conflict-free branches by direct refspec to avoid local merge commits.
• Add --dry-run mode: fetches and probes normally but skips all pushes and PR creation, printing what it would do instead (branch name, commit range, PR title, and conflicted files if applicable).
• Wire --dry-run to a boolean workflow_dispatch input so it can be toggled from the Actions UI.
• Use "conflict-free" instead of "fast-forward" throughout: the script merges rather than pointer-advances, so the old term was misleading.
• Activate the workflow: remove the if: false guard and enable the schedule and workflow_dispatch triggers. The pull_request trigger has been dropped in favour of relying solely on the 6-hour schedule.

Repository variables required before merge

• AUTOMERGE_TARGET_BRANCH — branch to merge into (e.g. amd-staging)
• AUTOMERGE_UPSTREAM_BRANCH — upstream branch to pull from (e.g. master)

Test plan

• Trigger manually from the Actions UI with dry-run checked — verify no pushes or PRs are created, output shows what would happen
• Trigger manually with dry-run unchecked — verify origin/master is updated and a conflict-free or conflict PR is opened
• Unit tests pass: python3 -m pytest .github/scripts/test_automerge.py

[lumachad]

Updated to do a general FF -> conflict-free merge renaming.

[spatrang]

Summary

Nice improvement — generic config, dry-run wiring, conflict-free naming, and enabling the workflow all look solid.

Action items before merge:

• Fix dry-run fall-through in §7b (bug)
• Confirm repo variables are configured
• Update tests/docs for renamed API (this PR or immediate follow-up)

---

Comments on files not changed in this PR

.github/scripts/test_automerge.py — renamed symbols

test_automerge.py still references removed/renamed APIs: find_open_ff_pr, open_ff_pr, push_master_mirror. Please update to find_open_conflict_free_pr, open_conflict_free_pr, and push_upstream_mirror in this PR or an immediate follow-up.

.github/scripts/test_automerge.py ~L203–221 — merge abort test expectation

test_cleanup_runs_even_when_abort_raises expects probe_clean_prefix to raise when merge --abort fails. This PR changed abort to check=False (automerge.py L378), so cleanup completes and the function returns instead of raising. Please update the test to assert cleanup still runs and (last_clean, first_conflict) is returned correctly.

.github/scripts/test_automerge.py ~L344–351 — missing dry-run regression test

TestDryRun.test_dry_run_partial_prefix_skips_push_and_pr mocks probe_result=("aaa", "bbb") but does not assert that conflict PR dry-run output is not printed. Once the §7b return is fixed, please add an assertion that only the conflict-free dry-run path runs for partial prefix.

.github/scripts/automerge.md — documentation out of date

Documentation still describes hardcoded amd-staging/master, "fast-forward" terminology, and old branch prefixes/constants. Please update to match configurable branches, conflict-free naming, dry-run, and required repo variables.

[spatrang]

Second-round review

All first-round items are addressed. Unit tests pass (19 tests).

First-round item	Status
§7b dry-run missing return	Fixed (L549)
Import-time env vars break tests	Fixed — _load_automerge() injects _TEST_ENV
Tests still used old APIs (find_open_ff_pr, open_ff_pr, push_master_mirror)	Fixed
Abort test expected a raise after check=False	Fixed — test_cleanup_runs_even_when_abort_fails asserts return values + cleanup
Dry-run partial-prefix missing conflict-path assertion	Fixed — mock_conflict_pr.assert_not_called()
automerge.md out of date	Fixed
Direct refspec push	Confirmed intentional
Broad pull_request trigger	Resolved by dropping the trigger; schedule + workflow_dispatch only
Repo variables must be set before merge	Confirmed (AUTOMERGE_TARGET_BRANCH=amd-staging, AUTOMERGE_UPSTREAM_BRANCH=master)

Pre-merge reminder (not a code change): set those two repository variables before the scheduled job starts running.

LGTM.