The no-op path worked (the else branch runs and exits 0), but using `git fetch`
as the existence check printed `fatal: couldn't find remote ref ...` to stderr
when the branch is absent -- which reads as a failure even though the step
succeeds. Probe with `git ls-remote --exit-code --heads` instead, which is
silent and sets a clean exit code; only fetch/checkout when the branch exists.
## Problem
The trial pipeline errored:
```
git clone -q --branch auto/registry-patch-proposals ...
fatal: Remote branch auto/registry-patch-proposals not found in upstream origin
```
I deleted the stale `auto/registry-patch-proposals` branch during cleanup, and — now that cascade detection (#128) makes the proposer correctly find **no candidates** — the proposer exits before ever recreating it. So the branch is legitimately absent, and the gate's hard `git clone --branch` fails.
## Fix
"Branch absent" = "no pending auto-patch proposals" = **nothing to verify**, which should be a clean no-op, not a failure. Clone `main`, then fetch + checkout the patch branch only if it exists; otherwise log and `exit 0`. A present branch is verified exactly as before (checkout its tip, diff registry vs `main`).
This also means the gate now runs `main`'s `trial-build-registry.R` (with the #130 fix) rather than a possibly-stale copy on the branch.
Reviewed-on: #132
Companion to bincraft codefloe #65. Sets `BINCRAFT_VERBOSE_PATCH_BUILD=TRUE` in the trial-build gate so a failed isolated patched build prints the real compiler error instead of `System command 'R' failed`. Harmless on bincraft versions without the flag (unknown env var is ignored). Takes effect once bincraft #65 is released and the build-env images pick it up.
Reviewed-on: #129
Closes the classifier loop (issue #115, step 3): turns the auto-proposable candidates into an actual PR, gated by a real trial build **in our own build-env images**.
Chosen model (from the design discussion): **autonomous PR**, **PR-first with a CI trial-build gate**, **bounded top-N batch** per run.
## Creating the patch PR
- `propose-patches.R` gains:
- `--limit N` -- act on the top-N candidates by failure volume; the rest defer to the next run (logged).
- `--open-pr` -- write the entries onto the reused `auto/registry-patch-proposals` branch, push (with `REPO_RW_TOKEN`), and open/update **one** PR via the Forgejo API (so re-runs update the same PR instead of piling up).
- `.crow/auto-apply-patches.yaml` -- a single job that runs `--open-pr --limit` on a cron/manual trigger. Needs `FORGEJO_TOKEN` + a write-scoped `REPO_RW_TOKEN`.
Novel source diffs and unknown signatures are still never proposed; nothing merges.
## The merge gate (our build-env images)
- `.crow/trial-build-registry.yaml` -- matrixed over the real `OS/IMG` build-env matrix (alpine:3.24, redhat:8/9/10, ubuntu:jammy/noble/**resolute** for ubuntu-2604). Each platform runs `local/trial-build-registry.R`, which diffs the branch registry against `main` and trial-builds **only the entries the branch adds** that apply to that platform, inside `reg.devxy.io/rpkgs/build-env-*`. Green only if every new entry builds; a platform with no new entries is a fast no-op. Nothing is uploaded/archived/recorded.
- The base-registry read **fails loud** if it can't read `registry.json` at `main`, rather than silently treating the base as empty and trial-building the whole registry.
## Notes / follow-up
- The repo uses **no `pull_request` triggers**, so the gate runs manually or on a cron against the branch (`--var patch_branch=...`). Wiring it to fire automatically on the PR needs `event: pull_request` enabled on the Forgejo webhook -- a one-line addition once that's on.
- Two new crons to register in the crow UI: `auto-apply-patches` and `trial-build-registry`. New secret needed: `REPO_RW_TOKEN` (write scope) for the push.
## Verification
- New pure helpers `entry_applies_to_os()` / `new_registry_packages()` covered by tests (platform codename/family/wildcard matching; added-vs-unchanged entry detection; per-platform filtering).
- `--limit` smoke (stubbed DB): top-2 by volume proposed, 3 deferred, candidate registry validates.
- Full suite: 105 tests pass; all pre-commit hooks pass (air, prettier, markdownlint, yamllint, validate-patches).
Reviewed-on: #124
