fix: recompute package snapshot when missing from cache (#95)

## Summary

`build-all.R` reads three snapshot files from `/mnt/cache/packages/` that the `build-all-versions-install-deps` step precomputes: `pkgs_to_build.rds`, `r_minor_sensitive_pkgs.rds`, and `s3_cache.rds`.
That cache volume is **per-agent**, so a build job scheduled on a different (fresh) agent than the one that ran install-deps finds the snapshot absent and dies at `readRDS` (`cannot open compressed file '/mnt/cache/packages/pkgs_to_build.rds'`).

This adds a conditional guard at the top of `build-all.R`: when any of the three files is missing, it sources `local/packages-to-build.R` (which has all needed creds via `PGPASS` / `B2_S3_*` env, already present in the build step) and writes the derived `.rds` files — exactly mirroring the install-deps command.

- The first build job on a fresh agent repopulates the shared cache, so subsequent jobs on that agent reuse it.
- Concurrent jobs that also miss simply redo the work (accepted tradeoff vs. slow shared storage like NFS).
- Saves use a temp-file + atomic `file.rename`, so a concurrent reader never sees a half-written `.rds`.

Reviewed-on: #95

local/build-all.R

@@ -15,6 +15,31 @@ library(bincraft, quietly = TRUE)
 library(future)
 plan("sequential")
 
+# The install-deps step precomputes the package snapshot into /mnt/cache, but
+# that volume is per-agent: a job landing on a fresh agent (or racing
+# install-deps) finds it empty. Recompute the snapshot here when any part is
+# missing, so the first job on an agent repopulates the cache for the jobs that
+# follow; concurrent jobs that also miss simply redo the work. Write via a
+# temp file + atomic rename so a concurrent reader never sees a half-written rds.
+package_cache_files <- c(
+  "/mnt/cache/packages/pkgs_to_build.rds",
+  "/mnt/cache/packages/r_minor_sensitive_pkgs.rds",
+  "/mnt/cache/packages/s3_cache.rds"
+)
+if (!all(file.exists(package_cache_files))) {
+  message("Package snapshot missing from cache; recomputing via packages-to-build.R")
+  dir.create("/mnt/cache/packages", showWarnings = FALSE, recursive = TRUE)
+  save_rds_atomic <- function(obj, path) {
+    tmp <- paste0(path, ".tmp.", Sys.getpid())
+    saveRDS(obj, tmp)
+    file.rename(tmp, path)
+  }
+  source(file.path("local", "packages-to-build.R"))
+  save_rds_atomic(pkgs, "/mnt/cache/packages/pkgs_to_build.rds")
+  save_rds_atomic(pkgs[r_minor_sensitive == TRUE], "/mnt/cache/packages/r_minor_sensitive_pkgs.rds")
+  message("Package snapshot recomputed.")
+}
+
 pkgs <- if (sensitive_only) {
   readRDS("/mnt/cache/packages/r_minor_sensitive_pkgs.rds")
 } else {