## Summary
Two changes:
1. **`OS`/`OS_VERSION` manual-run dropdowns** — give these form variables explicit `options:` lists (like `target_arch` and `R_VERSION`), so the manual-run form shows dropdowns instead of free-text, in both `build-all-versions.yaml` and `build-all-versions-install-deps.yaml`. Crow form variables are independent (no cascading), so the operator still has to pick a coherent `OS` + `OS_VERSION` combination (e.g. `redhat` + `9`, not `alpine` + `jammy`).
2. **Repairs `main`** — the crow fix from PR #96 (`bc2f6f1`) was lost when that PR was squashed (only the first commit was captured). As a result `main` currently carries the `OS: ${OS}` env vars that break Crow parsing (`unable to parse variable name`) and the unfixed `build-all.R`. This PR re-applies that fix: drop the env additions and derive `platform`/`arch` inside `build-all.R` from the container (bincraft codename → platform mapping + `Sys.info()` arch).
## Notes
- `OS_VERSION` options are quoted strings so tags like `8`/`9`/`10` aren't parsed as integers.
- Validated: both YAMLs parse, `build-all.R` parses.
Reviewed-on: #97
pat-s
83c761b736