Adds the scheduled pipeline for the failure-triage proposer/tracker landed in #117 (issue #115, steps 3 + 4).
## `.crow/weekly-patch-proposals.yaml`
A single (non-matrix) job -- the classifier groups over all of `single_builds`, so unlike the per-platform `weekly-audit-missing` it runs once globally. Weekly it:
1. `propose-patches.R --open-issue` -- posts/updates a Forgejo tracking issue with the auto-proposable registry entries (human reviews, trial-builds, opens the PR; nothing merges).
2. `proposal-tracking.R` -- logs the feedback-loop metrics (signature hit rate, proposed-vs-merged, retirement candidates).
Low blast radius: clones read-only (`REPO_RO_TOKEN`); the only write is the tracking issue via `FORGEJO_TOKEN`. It builds nothing (no B2 keys, no bincraft).
Triggers: `cron: weekly-patch-proposals` (register the cron in the crow UI) or manual with `task=weekly-patch-proposals`, matching the `archive-missed-packages` convention. Documented under "Scheduled run" in `local/patches/README.md`.
`yamllint` and the other pre-commit hooks pass.
Reviewed-on: #118
