Fix stale custom-source redeploys on quick-update (#2399)

* fix(installer): refresh custom-source cache on quick-update and persist channel marker

* fix(installer): persist real next ref and atomically dedupe custom refresh

* fix(installer): preserve custom-source cache when remote unreachable

When git fetch fails against an existing custom-module cache, cloneRepo
previously wiped the cache and attempted a fresh clone, which then also
failed for the same reason (network down, repo deleted/moved, auth
revoked) — leaving the user with no usable cache. With the new
quick-update refresh path calling cloneRepo for every cached custom
module, this turned transient remote outages into cache loss on every
quick-update.

- cloneRepo: on fetch failure with an existing cache, keep the previous
  clone and surface a warning via prompts.log.warn instead of removing
  the cache. The downstream metadata write uses the existing HEAD.
- _refreshRepoCacheOnce: update the comment to reflect that the common
  "remote unreachable but cache exists" case is now handled inside
  cloneRepo; warn on the remaining unrecoverable failures so they
  aren't silent.

Tests: 349 passed, 0 failed.

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Co-authored-by: Brian Madison <bmadcode@gmail.com>